Add support for sitemap checking

Sitemaps are pretty important and it’s nice to check they are functioning, i.e. perform a number of well-formed document checks, particularly when they are auto-generated. However, BugBug (not unreasonably) waits for the document readyState before checking a page, however, readyState is a part of the HTML DOM which doesn’t exist in the processing of a application/xml response, meaning checking appears impossible. I suspect a bit more of a difficult one?