Asset whitelist, Sprockets 3 compatibility.

This PR combines a couple of fixes: one to whitelist specific file extensions---or, really, files matching any regular expression, as it's meant to be user-configurable---and one to make the gem's asset-finding logic compatible with Sprockets 3.

Incidentally, if you're looking for volunteers to help maintain the gem in the future, I'd be happy to throw my hat in the ring. My e-mail address is in my GitHub profile if you'd like to chat about it directly. I'd hate to see this gem languish, if only for the selfish reason that I'd like to keep using it, and I have some ideas for improving it---adding tests, making that `Configuration` object idiom standard, etc.