Allow regex-wise selection of files.

There is clearly interest in ignoring certain files from the watcher: <https://github.com/stefanpenner/ember-cli/issues/2329>.
Indeed, certain files such as vim's (and emacs') save files such as `.swp`, or `.DS_Store` files on MacOS X machines, will trigger extra rebuilds from ember-cli at an alarming rate.

This patch is a first step towards allowing restrictions on which files trigger a rebuild in ember-cli.