Add sizing functions (like \small)

Summary:
Right now, when the size gets bigger, this still doesn't work, so there's a
check to prevent that. However, functions that go smaller (like `\small`,
`\tiny`, etc) do work. Also, we can't seem to use the sizing functions inside
of fractions (so something like `\dfrac{\small\frac{x}{y}}{z}` doesn't work).
However, the most prominent use case is `\small` as the outer-most object, so
this is still helpful. This commit has the parsing and stuff to handle all of
it, but it'll throw an error if you try to do something that doesn't work. (For
the record, "doesn't work" means "looks bad", not "throws an unexpected
error").

Test Plan:
Make sure things like `\small x` work, and things like `\Huge x` and
`\frac{\small x}{y}` don't.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D3619