Fix \binom and \choose

[jared-hughes]

Mathquill supports:

• Binomial coefficient \binom{}{} analogous to Fraction \frac{}{}
• \choose almost analogous to typing / to create a fraction, but more analogous to \over (which also works in Mathquill)

This PR makes three changes:

• Fixes bug where typing "choose" gave a TypeError (prototype was called without .call)
• Allows Up and Down keys to navigate a binomial.
• Adds better aria support.

For all three changes, the tests are copy-pasted from the fraction tests with minor changes.