Add support for .jsx files

This adds support for uglifying `.jsx` files, which are transformed to regular JavaScript using [reactify][]

I noticed that there have been many patches to add extensions to this regex. Could the test be more liberal? Perhaps `extension !== ".json"`? This seems to work for my use case. I would be happy to send a patch if you think this would work.

[reactify]: https://www.npmjs.org/package/reactify