Support for grunt target "files" #159

Please don't accept this right away - I would like to discuss this and there is still more to do.

This pull request provides initial support for the `files` Grunt target option.  I have only implemented the `src` half of it.  `dest` is next on my list before this is ready to accept.

The new test `multifiletest` seems to work fine.

I think it will be a lot of work to make this work with `fast` because we'd have to track separate cache timestamp files, so at least initially I'd like to defer that capability and assume `fast: 'never'` if someone uses `files`.

Any feedback is welcome and I am willing to rename stuff / refactor as needed.