Add code for generating HTML

Test Plan: Ran unit tests. Looked at `\blue{\displaystyle \left(\dfrac{a^\sigma}{\sin \theta}\right\Updownarrow \intop_{1/2}^{z^z} \sum_{i=0}^\infty x \,dx}` in Chrome and saw the future in my eyes.

Reviewers: emily

Reviewed By: emily

Subscribers: jessie

Differential Revision: http://phabricator.khanacademy.org/D13154