Allow TemplateHooks to choose how they want to render contents.

TemplateHooks could previously only supply a template_name, and nothing
else, for rendering. This severely limited them.

Now, rendering is handled in TemplateHook's render_to_string function.
By default, this will simply render the provided template, but an
extension can subclass TemplateHook and override this.

Also, template_name is now an optional parameter.

Tested with an extension I'm developing. I was able to provide my own
logic.