code: public function getConfigTreeBuilder()
      {
          $treeBuilder = new TreeBuilder();
          $treeBuilder->root('rss_client')
              ->children()
                  ->arrayNode('cache')
                      ->addDefaultsIfNotSet()
                      ->children()
                          ->scalarNode('adapter')
                              ->defaultValue('Desarrolla2\Cache\Adapter\NotCache')
                              ->cannotBeEmpty()
                          ->end()
                          ->scalarNode('ttl')
                              ->defaultValue('3600')
                              ->cannotBeEmpty()
                          ->end()
                      ->end()
                  ->end()
                  ->arrayNode('processors')
                      ->prototype('scalar')
                      ->end()
                  ->end()
                  ->arrayNode('channels')
                      ->useAttributeAsKey('id')
                      ->info('RSS Client Channels Configuration')
                      ->treatNullLike(array())
                          ->prototype('array')
                          ->treatNullLike(array())
                              ->prototype('scalar')
                              ->treatNullLike(null)
                              ->end()
                          ->end()
                      ->end()
                  ->end()
              ->end()
              ;
          return $treeBuilder;
      }
docs: {@inheritDoc}