Add webmention support

![](http://dissociatedpress.com/wp-content/uploads/2011/02/oh-the-huge-manatee.jpg)
[Not currently](https://github.com/jgarber623/FrancisCMS/blob/add-webmention-support-v2/db/migrate/20141022184758_create_webmentions.rb), no. Worth adding one?