Sitemap post not deleted when all posts deleted

[Mobius5150]

When the last post on a given day is deleted the underlying post is not removed and may still contain the post data -- meaning it will be served if requested.

We should probably hook into the deleted_post action and check if this is the last sitemap for that day. If it is we can go ahead and delete the post (takes very little time). If it isn't then we should flag that post to be updated.

[nickdaugherty]

This comes from this check in the cron and cli commands:

if ( Metro_Sitemap::date_range_has_posts( $date_stamp, $date_stamp ) ) {
    Metro_Sitemap::generate_sitemap_for_date( $date_stamp );
}

Needs #69 and #70 to be resolved.