merge Aj argparse branch into master

argparse material for "Parsing Command Line Arguments" section of the course

_episodes/04-argparse.md

@@ -353,7 +353,7 @@ keypoints:
 {: .challenge}
 
 > ## Options & Arguments Revisited
-> In an earlier exercise (),
+> In an [earlier exercise](#options--arguments),
 > you filled in the blanks in code designed to
 > parse options and arguments from the command line using `sys.argv`.
 > Use what you've learned about `argparse` to write a parser to replace that