Load `.jshintrc` files

Re. https://github.com/plumberjs/plumber-jshint/issues/2

`RcLoader` already does some work to cache files, but we might be able to
improve upon the performance. In any case it would be good to ship this!
I suspect it may be quite inefficient to do this for every single resource. I don't mind sacrificing performance for correctness here, but if you think that is true, can you add a TODO?