Auto-Sitemap - add layout option to include custom content which is not yet supported by the auto-sitemap itself

While you can edit the page that the custom sitemap is included on, since the sitemap generates the XML boilerplate, any custom content is difficult to hoist inside of that- and ends up outside the root of the XML document.

We should add an option for inserting custom links into the end of the sitemap after the auto-generated content.