Mustache_Autoloader should only be used as fallback.

If Composer is already handling autoloading the Mustache library, no need to require and register Mustache_Autoloader.
Only instantiate a single Mustache_Engine.
Don't manually load template contents.

Use a \Mustache_Loader_FilesystemLoader instance instead. This saves us some code, and makes it so loading partials works too :)
Allow specifying \Mustache_Engine configuration.
Use default loader for partials if none specified.