Update Mustache View implementation.

 * Only `require_once` and use the Mustache autoloader if Composer autoloading isn't already present.
 * Instantiate a single Mustache engine instead of one per `render` call.
 * Use a Mustache template loader rather than manually loading template contents.
 * Use a Mustache template loader for partials, which makes them actually work :)
 * Allow passing options to the Mustache engine. This is especially important for things like the `cache` option, which makes Mustache holy bacon faster than without it.