Since we're going for a RESTful micro-framework, I'm not sure `sleepy` should provide the ability to add non-resources to the API.

I'm not against adding this functionality, but maybe a cleaner fix here is just to change `mux()` to `Mux()` to export it. This would give consumers of `sleepy` the ability to add whatever they want to the mux, without explicitly needing to providing them the helper function to do so.