Adding a reviewer once a review has already started

[arfon]

Currently if we need to swap out a reviewer mid-review, then Whedon can only accomplish part of this:

@whedon add @newreviewer as reviewer

☝️this command does invite the new reviewer as a collaborator but doesn't make them their own checklist to work through.

It would be good if we could find a way to safely (i.e. without accidentally clobbering old checklists) add a checklist for a new reviewer mid-review.