title: Allowing extra, undefined options in a config array when using Symfony2's Configuration class
body: <p>I'm trying to define a <code>Configuration</code> object. I have successfully defined an array prototype node (like <code>security.firewalls</code>). My prototye array has an required element but I want to allow arbitrary parameters to be added to each array if needed.</p>
  
  <p>My question is, how can I allow extra, undefined elements to be added to each prototype array?</p>
  
  <p>Here's my config:</p>
  
  <pre><code>acme_widget:
      handlers:
          handler_one:
              service: handler.one.service
          handler_two:
              service: handler.two.service
              extra_array:
                  - Extra 1
                  - Extra 2
              extra_scalar: Extra 3
  </code></pre>
  
  <p>Here's my class builder:</p>
  
  <pre><code>    /**
       * Generates the configuration tree.
       *
       * @return TreeBuilder
       */
      public function getConfigTreeBuilder()
      {
          $treeBuilder = new TreeBuilder;
          $rootNode = $treeBuilder-&gt;root('acme_widget');
  
          $rootNode
              -&gt;children()
                  -&gt;arrayNode('handlers')
                      -&gt;useAttributeAsKey('service')
                      -&gt;prototype('array')
                          -&gt;children()
                              -&gt;scalarNode('service')-&gt;isRequired()-&gt;end()
                              -&gt;booleanNode('enabled')-&gt;defaultTrue()-&gt;end()
                          -&gt;end()
                      -&gt;end()
                  -&gt;end()
              -&gt;end()
          ;
  
          return $treeBuilder;
      }
  </code></pre>
  
  <p>I'm getting "InvalidConfigurationException: Unrecognized options".</p>