See #61 (and https://support.google.com/webmasters/answer/178636?hl=en)
Adds the post thumbnail as an image:image child to each sitemap entry (if images have not already been added to the entry).
Other things that might be nice to have:
Non-static method Metro_Sitemap::add_entry_featured_image() should not be called statically
Better variable name for sitemap entry xml
Add thumbnail image as image:image entry
Adds back-compat handling and bails if we already find existing images
added to the sitemap entry.
Allow images to be filterable
via the `msm_sitemap_entry_images` filter