Add message-editing hook

This allows us to write a plugin that automatically prefills CCs into
the pull request description.  Importantly, it happens *before* the user
gets the message opened in their editor, so they can still edit it (it's
on the same level as the pre-fill from the last commit).
Merge add-pull-request-description-hook (#124) into master

Add message-editing hook

This allows us to write a plugin that automatically prefills CCs into
the pull request description.  Importantly, it happens *before* the user
gets the message opened in their editor, so they can still edit it (it's
on the same level as the pre-fill from the last commit).