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
`comment.trim().indexOf('jscs: ') === 0`

I plan on having `jscs:enable`, `jscs:disable`, `jscs:enable:ruleName`.

Checking for the `: ` will disambiguate.