Transform .handlebars files too.

There are a few different extensions used for Handlebars templates, but .handlebars is the [default in the CLI](https://github.com/wycats/handlebars.js/blob/7538b8556c6e7875dcb5fe7cfff945d7a4da0bd0/bin/handlebars#L68-L73).

Would be nice to include these as well, there's been a couple of times I've used this and had to rename our existing templates. Thanks! :)