Allow JSHint chaining

Files that already contain JSHint gulp plugin errors will not be analysed by this JSLint gulp plugin. This allows them both to be chained together.
They are equivalent. [Documentation](http://nodejs.org/api/stream.html#stream_transform_transform_chunk_encoding_callback) doesn't spell it out, but the callback is following node convention that is used everywhere else: `cb(err, value)` (documentation only mentions `cb(err)`.