Provide template filename to Sinatra/Tile via settings.assets

Compilation errors for dynamic templates always show `class_methods.rb` as the filename. This is because the `filename` option passed to `render` is never used (presumably it used to work, but it no longer does).

However per the commits below, it is possible to supply the template filename via `settings.templates`. This is something of an abuse of the intent of `settings.templates`, but given the benefit of showing the correct error information I think it's worth it.