Add support for \{,d,t}binom

Test Plan: `\binom xy^{\binom xy^{\binom xy}}` looks like something. `\dbinom` and `\tbinom` also seem to work.

Reviewers: emily

Reviewed By: emily

Subscribers: jessie

Differential Revision: http://phabricator.khanacademy.org/D13315
Add \choose support

Test Plan: Looked at `1 + 2\choose {3 \over 4}`, npm tested.

Reviewers: emily

Reviewed By: emily

Differential Revision: http://phabricator.khanacademy.org/D13903
Update description in package.json
Release 0.1.1
Add braces around displaystyle

This makes a difference for expressions like `2 \choose 3`.

Auditors: emily