added a way to post configure Twig (useful to lazy-register Twig extensions for instance)

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(...);
    })
));

src/Silex/TwigExtension.php

@@ -18,6 +18,9 @@ class TwigExtension implements ExtensionInterface
         $app['twig'] = $app->share(function () use ($app) {
             $twig = new \Twig_Environment($app['twig.loader'], $app['twig.options']);
             $twig->addGlobal('app', $app);
+            if (isset($app['twig.configure'])) {
+                $app['twig.configure']($twig);
+            }
 
             return $twig;
         });