title: Create subform and append it in exisiting form in symfony 3
body: <p>I have this configuration : </p>
  
  <pre><code>ModuleModelType
  | - title
  | - created
  | - subform
  | - DevField1 
  | - DevField2 
  | - etc... 
  </code></pre>
  
  <p>What I want to have is something like this : </p>
  
  <pre><code>ModuleModelType
  | - title
  | - created
  | - subform
  |   | - DevField1 
  |   | - DevField2 
  |   | - etc... 
  </code></pre>
  
  <p>But the problem is that i don't know what users will add to the form, as I'm creating a reusable bundle and I don't want name conflicts.</p>
  
  <p>This is my code, how I can make this easily ? 
  I want a developer can create a class and append his own form in an existing form.The following existing code append the dev's form into the first one and can replace some existing keys if keys are the same. 
  Actually, I want to append all dev's fields into and <code>subform</code> to avoid this.</p>
  
  <pre><code>&lt;?php
  // Into a controller
  /** @var ModuleInterface $instance */
  $instance = ... ;
  $form = $this-&gt;createForm(ModuleModelType::class);
  $subform= $this-&gt;createForm(new FormType());
  $instance-&gt;buildForm($subform);
  $form-&gt;add('subform',$subform); // I want to make something like that
  
  interface ModuleInterface
  {
      public function buildForm(Form &amp;$form);
  }
  
  class Foo implements ModuleInterface
  {
      public function buildForm(Form &amp;$builder)
      {
          $builder-&gt;add("DevField1", /* type */);
          $builder-&gt;add("DevField2", /* type */);
      }
  }
  
  class ModuleModelType extends AbstractType
  {
      public function buildForm(FormBuilderInterface $builder, array $options)
      {
          $builder
              -&gt;add('title')
              -&gt;add('created')
              -&gt;add('subform')
          ;
      }
  }
  </code></pre>