Allow extensions to set the body of the new comment editor

[alexr00]

This would enable GitHub's "suggestion" feature in the GitHub Pull Request and issues extension:

1. When reviewing some's change, you want to leave a comment that contains a suggested change.
2. You hit the + to create a comment.
3. You write a message in the comment editor, then hit a button in the comment widget which appends a GitHub style suggestion block text to your in-progress comment.

[alexr00]

This can be done by exposing the comment editor as a real editor in the extension API.

[alexr00]

FYI @hermannloose and @laurentlb since you may also be interested in this.