title: How to pass options to CustomType in `collection` field Symfony 2.1?
body: <p>I have <code>SuperType</code> Form for Entity <code>Super</code>.</p>
  
  <p>In this form I have a <code>collection</code> field of <code>ChildType</code> Form types for Entity <code>Child</code></p>
  
  <p><strong><code>class SuperType:</code></strong></p>
  
  <pre><code>public function buildForm(FormBuilderInterface $builder, array $options)
  {
      $builder-&gt;add('childrens', 'collection', array(
              'type' =&gt; new ChildType(null, array('my_custom_option' =&gt; true)),  
  }
  </code></pre>
  
  <p><strong><code>class ChildType:</code></strong></p>
  
  <pre><code>public function buildForm(FormBuilderInterface $builder, array $options)
  {
      if ($options['my_custom_option']) {
          $builder-&gt;add('my_custom_field', 'textarea'));
      }
  }
  
  public function setDefaultOptions(OptionsResolverInterface $resolver)
  {
    $resolver-&gt;setDefaults(array(
        ...
        'my_custom_option' =&gt; false
    ));
  }
  </code></pre>
  
  <p>How can I change the <code>my_custom_option</code> value only for this <code>SuperType</code> form?</p>
  
  <p>Of course, what I've tried passing this option via constructor doesn't work.</p>