Parse single superscripts and subscripts

Summary:
Add the ability to parse lone `^x` and `_y`, etc. This basically just
involves more checking of cases in the increasingly ugly `parseAtom` function.
Also, now we manually check for the cases of double superscripts and
subscripts.

Test Plan: Make sure the tests pass. Make sure things like `^x` and `_y` parse.

Reviewers: emily

Reviewed By: emily

Differential Revision: http://phabricator.khanacademy.org/D3095