Render Widgets

First, this PR renames the concept formerly known as a 'module' to 'widget'.

It also adds the ability to (almost) use whatever templating engines are available in their Rails application when registering widgets. It currently works for ERB, HAML, and just plain HTML, although the only thing limiting this is the `show` action (which has to determine what the correct widget file is by its basename).

Because I extracted the template for a widget into its own partial, I also added a new `widgets/` directory for user-defined widgets.

I could polish this PR for a while but I'd rather use this PR for a discussion about style and concepts, @begriffs.