Add `footer` support to `blockquote`

According to http://www.w3.org/html/wg/drafts/html/master/grouping-content.html#the-blockquote-element, the blockquote attribution in the HTML5 spec should be within a footer. Within that, specific references to bodies of works should be within cite elements. However, it also appears cite on it's own will suffice (without the footer).

This breaks backward compatibility if we go right into it, so we can't do that. #11408 got us close with the cite element, but that leaves out the footer element.

Whatever solution we implement should account for all of this, and update the docs accordingly (with examples and links to the above spec).