Add support for .ember.hbs extension, so Rails won't generate funny URLs

A fix for multiple framework support in Rails, when not using haml or slim. You can now use a `.ember.hbs` file extension, e.g. `some-cool-template.ember.hbs`.

Colophon: Currently, it seems that if you're not using haml or slim, the files have to have a `.hbs.ember` extension to get the multiple framework support to work. Unfortunately, in dev environment, Rails creates the URLs to files (templates) with a `.ember.js` extension, which doesn't match. So Rails returns a 404, because it generated a funky path.

:hamburger: 

PS. handlebars_assets still accepts files with just `.ember` extension, eventho' it doesn't seem to work atleast in Rails.