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
> I don't need to test that the config only affects a single file because a new JsFile object is created per call of checkString.

But that's an implementation detail. I still think you should test that the config only affects one file. You know it'll pass, but it'll prevent possible future regressions. 

> I don't need to test that files without the config work, because the entire suite passes.

Agreed.