Mathjax support added

Mathjax config options added.
`compress_html` config option changed,
Mathjax script loaded by head when `page.mathjax` is truthy.
Documentation and test page added.
See also issue #198.

_config.yml

@@ -50,5 +50,10 @@ compress_html:
   comments: all
   endings: all
   startings: []
-  blank_lines: false
+  blanklines: true # required for Mathjax
   profile: false
+
+# For the Mathjax script loaded in the head of all pages with page.mathjax
+mathjax:
+  source: https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.5/latest.js?config=TeX-AMS_CHTML
+  config: 'MathJax.Hub.Config({ TeX: { equationNumbers: { autoNumber: "AMS" } } })'

_includes/head.html

@@ -29,7 +29,17 @@
   {% if site.search_enabled != false %}
     <script type="text/javascript" src="{{ '/assets/js/vendor/lunr.min.js' | absolute_url }}"></script>
   {% endif %}
-  <script type="text/javascript" src="{{ '/assets/js/just-the-docs.js' | absolute_url }}"></script>
+  
+    <script type="text/javascript" src="{{ '/assets/js/just-the-docs.js' | absolute_url }}"></script>
+  
+  {% if page.mathjax %}
+    <script type="text/x-mathjax-config">
+      {{ site.mathjax.config }};
+    </script>
+    <script type="text/javascript" async 
+      src="{{ site.mathjax.source }}">
+    </script>
+  {% endif %}
 
   <meta name="viewport" content="width=device-width, initial-scale=1">
 

docs/configuration.md

@@ -89,3 +89,13 @@ See [Customization]({{ site.baseurl }}{% link docs/customization.md %}) for more
 # e.g, UA-1234567-89
 ga_tracking: UA-5555555-55
 ```
+
+## Mathjax
+
+```yaml
+compress_html:
+  blanklines: true # required for Mathjax
+mathjax:
+  source: https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.5/latest.js?config=TeX-AMS_CHTML
+  config: 'MathJax.Hub.Config({ TeX: { equationNumbers: { autoNumber: "AMS" } } })'
+```

docs/mathjax/index.md

+---
+layout: default
+title: Mathjax
+nav_order: 8
+has_children: true
+mathjax: true
+---
+
+# Mathjax
+
+To use [Mathjax] on a just-the-docs page:
+
+- include `mathjax: true` in the YAML preamble
+
+- enclose $$ \LaTeX $$ code in [kramdown math blocks]
+
+The default Mathjax options in `_config.yml` are:
+
+```yaml
+compress_html:
+  blanklines: true # required for Mathjax
+mathjax:
+  source: https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.5/latest.js?config=TeX-AMS_CHTML
+  config: 'MathJax.Hub.Config({ TeX: { equationNumbers: { autoNumber: "AMS" } } })'
+``` 
+
+Here is an example:
+
+$$
+\begin{align*}
+  & \phi(x,y) = \phi \left(\sum_{i=1}^n x_ie_i, \sum_{j=1}^n y_je_j \right)
+  = \sum_{i=1}^n \sum_{j=1}^n x_i y_j \phi(e_i, e_j) = \\
+  & (x_1, \ldots, x_n) \left( \begin{array}{ccc}
+      \phi(e_1, e_1) & \cdots & \phi(e_1, e_n) \\
+      \vdots & \ddots & \vdots \\
+      \phi(e_n, e_1) & \cdots & \phi(e_n, e_n)
+    \end{array} \right)
+  \left( \begin{array}{c}
+      y_1 \\
+      \vdots \\
+      y_n
+    \end{array} \right)
+\end{align*}
+$$
+
+For further examples, see [this test page](test), adapted from a [test]
+in [markdown-preview-plus].
+
+[Mathjax]: https://www.mathjax.org 
+  "Mathjax home page"
+
+[kramdown math blocks]: https://kramdown.gettalong.org/syntax.html#math-blocks
+  "kramdown math blocks documentation"
+
+[test]: https://github.com/atom-community/markdown-preview-plus/blob/master/EXAMPLE.md
+  "Markdown-preview-plus test file"
+
+[markdown-preview-plus]: https://atom.io/packages/markdown-preview-plus
+  "Markdown-preview-plus package page"

docs/mathjax/test.md

+---
+layout: default
+title: Mathjax test
+parent: Mathjax
+mathjax: true
+---
+
+$$x$$ math at the start of the document
+
+Nested: $$M = \text{while e^2 $e^2$ do $c^2$ end}$$
+
+**Numbered equations**
+
+$$
+\begin{equation}
+\int_0^x \sin(x) dx
+\label{eq:test}
+\end{equation}
+$$
+
+$$\eqref{eq:test}$$
+
+**Blockquotes**
+
+> 1.  A block quote
+> 2.  With a list inside
+> 3.  With $$math$$
+
+**Math environment Syntax testing**
+
+Inline Math should work with `$$`  $$\frac{x+y}{y}$$
+but not `\(` \(\frac{x+y}{y}\)
+
+Use `$$` on separate lines:
+
+$$
+\int_{-\infty}^\infty e^{-x^2} = \sqrt{\pi}
+$$
+
+
+Use `$$` on the same line:
+$$ \frac{-b \pm \sqrt{b^2-4ac}}{2a} $$
+
+Use `$$` on separate lines:
+
+$$
+a+b
+$$
+
+Use `$$` on the same line: $$E=mc^2$$
+
+----
+
+**Some Tex Functions**
+
+$$k \times k$$, $$n \times 2$$, $$2 \times n$$, $$\times$$
+
+$$x \cdot y$$, $$\cdot$$
+
+$$\sqrt{x^2+y^2+z^2}$$
+
+$$\alpha \beta \gamma$$
+
+$$
+\begin{aligned}
+x\ &= y\\
+mc^2\ &= E
+\end{aligned}
+$$
+
+----
+
+**Escaped Math environments**
+
+In the following examples no math environments should render:
+
+a $5, a $10 and a \$100 Bill.
+
+All McScrooge sees
+
+\$$
+
+and even more:
+
+\$$
+
+\\[x+y\]
+\\(x+y\)
+
+```
+$x
+```
+
+```
+\$
+```
+
+`\$`, `\[ \]`, `$x$`
+
+----
+
+**Testing `\newcommand`**
+
+For all $$x$$ and $$y$$ in $$\mathbb{R}^k$$ it is true that
+$$
+\newcommand{sca}[1]{\langle #1 \rangle}
+|\sca{x,y}|^2 \le \sca{x,x} \cdot \sca{y,y},
+$$
+
+<!-- newcommand not in environment-->
+\newcommand{\scalong}[1]{(#1_1,\dots,#1_k)}
+
+where $$\sca{\cdot,\cdot}$$ denotes the inner product $$\sca{\scalong{x}, \scalong{x}} = \sum_{i=1}^k x_i y_i$$.
+
+----
+
+**Testing several math fonts**
+$$
+p(\mathbf{m}) \sim \mathcal{N}(\mathbf{m}) e^{-\sum_{i} \beta_i m_i}
+$$
+
+$$
+\langle \vec{m} \rangle =
+\frac{1}{Z(\vec{\beta})}\vec{m}(\mu)
+\sum_{\mu\in\mathcal{G}}
+e^{-\vec{\beta}\cdot\vec{m}(\mu)}
+$$
+
+----
+
+Notice below that the `$$`-separated display math doesn't have a blank line before and after (i.e. in an actual $$\LaTeX$$ document it wouldn't be placed in a separate paragraph):
+
+The _characteristic polynomial_ $$\chi(\lambda)$$ of the
+$$3 \times 3$$ matrix  
+    $$
+\left( \begin{array}{ccc}
+a & b & c \\
+d & e & f \\
+g & h & i \end{array} \right)
+$$  
+is given by the formula  
+    $$
+\chi(\lambda) = \left| \begin{array}{ccc}
+\lambda - a & -b & -c \\
+-d & \lambda - e & -f \\
+-g & -h & \lambda - i \end{array} \right|.
+$$
+
+This is a long line: $$\chi(\lambda) = a e i-a e \lambda -a f h-a i \lambda +a \lambda ^2-b d i+b d \lambda +b f g+c d h-c e g+c g \lambda -e i \lambda +e \lambda ^2+f h \lambda +i \lambda ^2-\lambda ^3$$
+
+----
+
+**Does it work in different MD elements?**
+
+# Math $$x^2$$ in heading 1
+
+## Math $$x^2$$ in heading 2
+
+### Math $$x^2$$ in heading 3
+
+#### Math $$x^2$$ in heading 4
+
+$$x_1, x_2, \dots, x_N$$ should not conflict with `_` _italics_?
+
+$$(f*g*h)(x)$$ should not conflict with `*` *syn***tax**?
+
+$$[a+b](c+d)$$ should not conflict link [syntax](#)?
+
+_math $$x^2$$ in emphasis_
+
+**math $$x^2$$ in bold**
+
+[math $$x^2$$ in link](http://www.mathjax.org/)
+
+`math $$x^2$$ in code`
+
+~~math $$x^2$$ in strikethrough~~ (doesn't strike through the math)
+
+**In Tables**
+
+| Left-Aligned  | Center Aligned  | Right Aligned |
+| :------------ |:---------------:| -----:|
+| $$a+b$$       | some wordy text | $1600 |
+| $$a+b$$       | some wordy text | $1600 |
+| $$a+b$$       | some wordy text | $1600 |
+
+The following doesn't work (it only works with Pandoc)
+
+| Left-Aligned  | Center Aligned  | Right Aligned |
+| :------------ |:---------------:| -----:|
+| $$a+b$$       | some wordy text | $1600 |
+|               | $$
+A=\mathbf{A}=\underline{A}=\begin{pmatrix}
+a_{11} & a_{12} & \cdots & a_{1n}\\
+a_{21} & a_{22} & \cdots & a_{2n}\\
+\vdots & \vdots &        & \vdots\\
+a_{m1} & a_{m2} & \cdots & a_{mn}\\
+\end{pmatrix} = (a_{ij})
+$$                                |   $12 |
+|               |                 |    $1 |
+
+
+**In Image Captions**
+
+The `alt` string doesn't work
+
+![$$a^2+b^2=c^2$$](https://raw.githubusercontent.com/atom-community/markdown-preview-plus/master/imgs/mpp-full-res-invert.png)