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
Seems it would require more tests than that, also do we really need a whole file for this? Check <a href="https://github.com/mdevils/node-jscs/blob/master/test/rules/require-multiple-var-decl.js#L60-69">this</a> out