Twig: adding twig extension debug from config file

Twig: adding twig extension debug from config file

According to TWIG documentation, in order to use the dumb function I
should first add the extension debug like so:
$twig = new Twig_Environment($loader, array(
'debug' => true,
// ...
));
$twig->addExtension(new Twig_Extension_Debug());
But how do I do that from config.yml ? I use symfony2.