Edit attribution with CriticMarkup ("Blame")

Right now CriticMarkup does not support a separate tag for assigning attribution to individual edits. We suggest using the comment tag and prepending an authors name with an @ symbol.

For example:

 Don't act so surprised, Your {~~Queen~>Highness~~}{>>@gsw sounds better<<}. 
 You weren't on any mercy mission this time.

This will be the most compatible with future changes in CriticMarkup. Correct attribution is difficult in a multi-editor context. We don't want to screw it up. Comments are unlikely to change in the future even if a new attribution tag is added.