title: Id attribute on form tag symfony
body: <p>I would like to define a id attribute on my symfony2 forms.</p>
  
  <p>I've tried with this in my twig template: </p>
  
  <pre><code>{{ form_start(form, {'id': 'form_person_edit'}) }}
  </code></pre>
  
  <p>But it seems not working.</p>