Add basic support for some environments in KaTeX. To create
multiple columns of objects, we simply stack vlists next to each other,
and then manually manage the vertical alignment ourselves.
So far, this only supports `matrix` (to an artibrary size, where real
LaTeX only supports 10 columns!), and the `align*` environment with only
two columns in it. Support for more environments shouldn't be terribly
hard with this basic structure though.
Partially fixes #61
- `make test`
- Make sure screenshots look good
Reviewers: kevinb, alpert
Differential Revision: https://phabricator.khanacademy.org/D17235