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 haven't had time to look into this, but added to `TODO` in https://github.com/OliverJAsh/plumber-jshint/commit/4be64149de4e3ef5af1fde249707456ae7c96158