Allow the creation of draft comments when reviewing pull requests

When reviewing code it would be useful to be able to create draft comments which are viewable only by the reviewer until such time they decide to post them.

This will allow you to make notes for yourself as you read down through the code or comments which you may want to edit prior to post or even completely discard - as they may change form as you further look through the code (especially for larger requests).

Comments as it stands the author gets a notification for each comment (and a notification for each subsequent edit of a comment) and may also immediately start to read/review them.