refactored to allow supplying a custom template rendering function

I refactored the template to supply the path of the template. The logic for rendering the template has been moved to the controller. The controller will default to using Antlers if no `render-fn` is specified. Otherwise, the renderer referred to by the `render-fn` will be used to render the template.

The `default-template` has been removed. It would be better to use middleware to return the params in `dev` mode than have it as a default behavior, as this potentially exposes any params keys set by middleware.

Updated `template` to throw an exception when the template cannot be found. Again, I think it would be better to handle exceptions using middleware.