Allow extension of FeedsSimplePieParser parsing

Obviously, a matching extension would need to be done to ::getMappingSources()

First attempt at a patch attached. This uses a protected method on FeedsSimplePieParser for the additional mapping.

Here's an example of an extension using the code from the patch. It overrides getMappingSources() to add the new mapping sources, and implements the new method provided by the patch to add parsing for these items.

/**
 * A simple parser that extends FeedsSimplePieParser by adding support for a
 * couple of iTunes tags.
 */
class MyParser extends FeedsSimplePieParser {
  /**
   * Add the extra mapping sources provided by this parser.
   */
  public function getMappingSources() {
    return parent::getMappingSources() + array(
      'itunes_keywords' => array(
        'name' => t('iTunes:Keywords'),
        'description' => t('iTunes Keywords.'),
      ),
      'itunes_duration' => array(
        'name' => t('iTunes:Duration'),
        'description' => t('iTunes Duration.'),
      ),
    );
  }

  /**
   * Parse the extra mapping sources provided by this parser.
   */
  protected function parseExtensions(&$item, $simplepie_item) {
    $itunes_namespace = 'http://www.itunes.com/dtds/podcast-1.0.dtd';
    if ($value = $simplepie_item->get_item_tags($itunes_namespace, 'keywords')) {
      $item['itunes_keywords'] = $value[0]['data'];
    }
    if ($value = $simplepie_item->get_item_tags($itunes_namespace, 'duration')) {
      $item['itunes_duration'] = $value[0]['data'];
    }
  }
}

Yes, I had a look - but that's aiming at something different - a completely generic+extensible XML parser. This patch simply aims to make the SimplePieParser extensible, since it already does 90% of what is needed.