Contexts should be able to alter the XML header output (and possibly link output?)

#764658: Use XMLWriter class instead of fwrite() is now probably something I want to look into more closely since it would *really* help this feature out.

So now I have this smart idea that SitemapTypes should just have to extend the XMLSitemapWriter class and that it should be pretty much automatic and we only have to handle which class is loaded to generate the XML:

For example, to generate a news sitemap:

class XMLSitemapNewsWriter extends XMLSitemapWriter {
  public function getRootAttributes() {
    $attributes = parent::getRootAttributes();
    $attributes['xmlns:news'] = 'http://www.google.com/schemas/sitemap-news/0.9';
    return $attributes;
  }

  public function writeSitemapElement($name, array &$element) {
    $element['news:news'] = array(
      'news:publication' => array(
        'news:name' => variable_get('site_name', 'Drupal'),
        'news:language' => $GLOBALS['language']->language,
      ),
      'news:title' => 'My Node Title',
    );

    return parent::writeSitemapElement($name, $element);
  }
}

Committed to CVS on both branches:
http://drupal.org/cvs?commit=413484
http://drupal.org/cvs?commit=413488