Usage example:

$app->register(new TwigExtension(), array(
    'twig.path'       => __DIR__.'/templates',
    'twig.options'    => array('cache' => __DIR__.'/../cache', 'debug' => true, 'strict_variables' => true),
    'twig.configure'  => $app->protect(function ($twig) use ($app) {
        $twig->addExtension(...);
    })
));