Treat files with custom extensions as JS

Fixes #27

Assume modules are JS unless they're explicitly named ".css". This is a
bit limiting in that you can't have other extensions for CSS modules,
but we already have to do custom work around CSS modules anyways so I'm
ok with that in this case.
Merge pull request #28 from tivac/issue-27-custom-extensions-as-js

Treat files with custom extensions as JS, Fixes #27