Summary:
Add an auto-render extension to render math on a page. It
exposes a global function (maybe we should attach it to `katex`?) to
render math in an element. It comes with a README on how to use it.
Also, make `make build` build the minified file.
Fixes#26
Test Plan:
- Visit http://localhost:7936/contrib/auto-render/
- See that all of the math renders correctly
- `make test`
Reviewers: alpert, kevinb
Reviewed By: kevinb
Differential Revision: https://phabricator.khanacademy.org/D16620