Support custom template- and rendering-engines

Hi Marc,
I've made some minor changes to let your module support custom rendering engines and template extensions:
In our case we're using the existing "$config->templateExtension" and set it to ".twig" (which wasn't supported by your module as you've hardcoded .php as the templates extension).
Furthermore I've changed the $parsedTemplate->render() call to render the page instead of just the template file as most modules hook to PageRender::renderPage to alter the templates output. Maybe this should be made configurable (for people using the "delegate approach")?