implement fileExtensionsToProcess config file option

Adds a `fileExtensionsToProcess` config option that lets the user specify an array of file extensions that should be processed.

My projects contain both `.js` and `.jsx` files that I want to run through jscs. This functionality makes it possible.
No. However, there aren't really any defaults listed anywhere in the README. It seemed to me that this particular option needed defaults explicitly stated.

I'm more than happy to present this information differently in the README. (And, you won't hurt my feelings at all if you just change it to whatever you want rather than kicking the PR back to me :smile: )