Default extensions to `.css`, `.html`, `.js`

[jescalan]

Right now we use jade, sss, and js. But you don't have to use sugarss, in fact it's only present in our base template as a configurable option. If we switch to posthtml, you won't necessarily need to use jade and it will parse any form of html.

That being said, I do think we need to make these extensions configurable. Right now we have the matchers config, but it might be worth adding an easier shortcut option like htmlExtensions: ['.html', '.jade'] etc. We need the extensions mostly for syntax highlighting and clarity when we're using a very different parser.

[kylemac]

i don't know if the shortcuts are any better. i need to think about that experience a little more