Add ability to canonify URLs in rendered XML output.

A fix for #725.

I'm not sure if I'm approaching this the right way. Please let me know what needs improvement.

Notes:

* Assumes that canonifying URLs in escaped HTML in regular (non xml) content is not desired.
* `AbsURLInXML` assumes content will be escaped with `| html`.
* Not sure if `AbsURLInXML` should also look for unescaped content.