Support .jshintrc

Original jSHint supports `.jshintrc`.
- http://jshint.com/docs/#usage
Sorry to my late response.

I add a commit to raise error explicitly. https://github.com/tricknotes/jshintrb/commit/bfd21b13262c0d38ec5bdcef5c287920e76b2cb1
Is this a way your intended?