Create new user or guest author while assigning to a post

[Originally from the forums](http://wordpress.org/support/topic/plugin-co-authors-plus-create-author-instantly), it would be neat if you could create a user or guest author on the fly while assigning to a post. For example, if you were to do a search for a user that doesn't exist, there would be a small prompt to create a new user/ guest author in a modal or the like.