Lint jsx files after they're transformed.

Summary:
This uses the jsx command to transform files then analyze the resulting
javascript. It will miss any legit lints that happen to be in the whitelist.

Test Plan:
* ./runlint ../webapp/javascript/clarifications-package
  - this outputs many single vs double-quote warnings
* ./runlint ../webapp/javascript
  - this outputs a mixed torrent of warnings

Reviewers: csilvers, alpert

Differential Revision: http://phabricator.khanacademy.org/D3111