Add TeX style support

Test Plan:
`\blue\frac12 + \frac{2(y-z)}{2+\frac1{7+\frac31}} \div \orange{\arctan x^{2+\frac43}_{2}} * 2^{2^{2^2}}` looks reasonable, as does
`\blue\frac12 + \dfrac{2(y-z)}{2+\frac1{7+\frac31}} \div \orange{\arctan x^{2+\frac43}_{2}} * 2^{2^{2^2}}`.

Reviewers: emily

Reviewed By: emily

Differential Revision: http://phabricator.khanacademy.org/D3047