added a Twig extension

Usage:

$app = new Application();
$app->register(new TwigExtension(), array(
    'twig.path'    => __DIR__.'/../templates',
    'twig.options' => array('cache' => __DIR__.'/../cache', 'debug' => true),
));

The "app" variable is registered as a global variable, so it is accessible in all templates:

$app['routing'] = new UrlGenerator(...);

{{ app.routing.generate('homepage') }}

src/Silex/TwigExtension.php

+<?php
+
+/*
+ * This file is part of the Silex framework.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Silex;
+
+class TwigExtension implements ExtensionInterface
+{
+    public function register(Application $app)
+    {
+        $app['twig'] = $app->asShared(function () use ($app) {
+            $twig = new \Twig_Environment($app['twig.loader'], $app['twig.options']);
+            $twig->addGlobal('app', $app);
+
+            return $twig;
+        });
+
+        $app['twig.loader'] = $app->asShared(function () use ($app) {
+            return new \Twig_Loader_Filesystem($app['twig.path']);
+        });
+    }
+}