SAT passage widget

Summary:
Passage widget for perseus vSAT! The widget is a textarea with markdown support and (optional) automatic line numbering. (The user must specifc their own line breaks.)

Also added underline support to our markedReact parser with double underscore syntax: __underline__ (this now underlines instead of bolds).

Test Plan:
- Add a passage widget and make an SAT reading question!
- Check that line-number toggling works and that you can't break anything.

Reviewers: eater, jack

Reviewed By: jack

Subscribers: marcos, alpert

Differential Revision: http://phabricator.khanacademy.org/D11744