Enabling twig dump function if DEBUG is defined

It would be handy to enable the twig dump function when DEBUG is defined. The debug setting is already passed through to the twig environment but it doesn't activate the dump function.

If you add this to ./phpbb/template/twig/twig.php near line 110:

if (defined('DEBUG')) {

   $this->twig->addExtension(new \Twig_Extension_Debug());

}

It enables this function: http://twig.sensiolabs.org/doc/functions/dump.html