title: add active class to link with sf2 and twig
body: <p>following simple code:</p>
  
  <pre><code>&lt;li&gt;&lt;a href="{{ path('_list') }}"&gt;List&lt;/a&gt;&lt;/li&gt;
  </code></pre>
  
  <p>is there a simple way to add an <code>class="active"</code> if the current page matches the <code>_list</code> route?</p>
  
  <p>using the newest PR-Release of symfony2 and twig as template engine</p>