Treat all unknown extensions as resources

Created by: gaearon

I’m getting a bit tired of extending our whitelist like in #665. I think we should have a “catch all” loader that uses file-loader for any unknown extensions.

This makes sense because it’s easy to explain: “if you import something that isn’t JS or CSS, it becomes a part of the build, you get its filename, and can do whatever with it”.

I think @dralletje planned to look into this.