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
Merge pull request #5 from hughsk/handlebars-extension

Transform .handlebars files too.