JsFile: Allows for file-wide jscs configuration 

Fixes #487
I'm iffy about this. Since the configuration syntax isn't well-formed JSON (i.e., the property names aren't quoted), you can't just `JSON.parse()` it to go from string to object.

The alternative (unless you guys have any suggestions) is a messy regex that identifies the property names – allowing me to loop over the matches and replace them with quoted versions.