Add underline support (double underscores)

Summary: Added underline support to the markdown parser with double underscore syntax: `__underline__` (this now underlines instead of bolds).

Test Plan: Check that __underline__ and **bold** display correctly

Reviewers: alpert, jack

Reviewed By: alpert, jack

Differential Revision: http://phabricator.khanacademy.org/D11803