Add support for .jshintignore

jshint cli exposes a method that can be used for to recursively look for  `.jshintignore` files and return files that are not ignored. I think this is the easiest way to achieve this. Feedback appreciated.
Doesn't seem to work. I tried adding `index.js` to `.jshintignore` and it still JSHinted it and `cli().gather({args: [file], cwd: path.dirname(file)})` returned the file.