title: Symfony Choice type with disabled options
body: <p>Is there any way with Symfony to render a <code>&lt;select&gt;</code> form type with disabled options, based on the truthyness of the given <code>choices</code> options ?</p>
  
  <p>I saw <a href="https://stackoverflow.com/questions/14344639/how-to-disable-specific-item-in-form-choice-type">this thread</a> (thanks to DonCallisto) about disabling choice expanded options ;
  However I do not want to have an expanded choice.
  I would like to keep a <code>select</code> element, with disabled <code>options</code>.</p>
  
  <pre><code>$builder-&gt;add('list', 'choice', array(
      'choices' =&gt; array(
          array(
              'value' =&gt; 1,
              'label' =&gt; '1',
              'disabled' =&gt; false
          ),
          array(
              'value' =&gt; 2,
              'label' =&gt; '2',
              'disabled' =&gt; false
          ),
          array(
              'value' =&gt; 3,
              'label' =&gt; '3',
              'disabled' =&gt; true
          )
      ),
      // Instead of
      // 'choices' =&gt; array(
      //     1 =&gt; 'Option 1',
      //     2 =&gt; 'Option 2',
      //     3 =&gt; 'Option 3'
      // )
  );
  
  # Which would render to the following element
  &lt;select [...]&gt;
      &lt;option value='1'&gt;1&lt;/value&gt;
      &lt;option value='2'&gt;2&lt;/value&gt;
      &lt;option value='3' disabled='disabled'&gt;3&lt;/value&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>I just can't find the way...
  Is it necessary to build its own field type ?</p>