Add support for Mustache templates.

I wanted support for templates and saw that there's an open issue for it (issue #42), so here is my attempt. This implementation is still a bit rough and I'd love to get some input on the design.

The biggest change is the introduction of the Handler struct. The Handler struct replaces the functions that handled the requests before. The Handler struct stores a mustache template and possibly also a function to operate on the template and request data. The updated examples look very similar and shouldn't be hard to understand.