perseus-markdown: Add perseus-markdown + mathy text

Summary:
Adds a "perseus-markdown" markdown variant, which supports
everything in simple-markdown and mathy text.

Test Plan:
 * `make test`
 * Replaced simple-markdown with perseus-markdown in the
   simple-markdown-tester widget (it's just a test widget--
   I don't feel too bad about leaving that naming for now),
   and tested adding math to output.

Reviewers: emily

Reviewed By: emily

Subscribers: alpert

Differential Revision: http://phabricator.khanacademy.org/D13412