JsFile: Allows for file-wide jscs configuration 

- [ ] replace the eval with a parser or munging the config into a JsFile instance then parsing the config's AST to get the keys and values

Fixes #487
Won't lint, so I'll replace it tomorrow. Going to try to construct a JsFile from the config to use the AST to get the keys and values to construct the object. Feels like the eval solution though. A regex might be the more "straightforward" solution.