Class Sitemap acts as the main container for the sitemap.xml file, setting
up the "urlset" attribute required for every sitemap.xml file and used to
hold URL containers and their encapsulated attributes.
Class SitemapOutput writes a urlset container to disk as a sitemap.xml file
(or a set of sitemap files referenced by a sitemap index file), in addition
to formatting the sitemap file(s) so that they are readable.