Add support for <r:page:edit_url/>

[adamjohnson]

Here's some HTML & Radius to test with:

Last updated on <a href="<r:page:edit_url/>"><r:date_format format="%B %-d, %Y" value="{page:updated_at}" /></a>.

Mock data:

page:
  updated_at: today
  edit_url: http://google.com

Nathan wants this to go into the standard footer at some point, so having the ability to use r:page:edit_url in hammer will be nice. For now, I'm adding this to the Honors College theme—which is the same theme we're going to use in the CleanSlate demo Thursday 1/15/15. He'd like for us to be able to say "Hey look! There's a typo, let's fix that." Then, we click this link in the footer and fix the typo. Quickest path to a simple fix.