Private pull request review comments/suggestions

I'd like to be able to direct 'private comments/suggestions' to a pull request review. Sometimes I want to make suggestions to the PR author without it appearing publicly. I often make suggestions in tools like Slack/Skype/etc to the author instead when a suggestion can be interpreted as "nit-picky" or could potentially make me appear like "know-it-all" if done publicly.

All comments in this thread should be invisible to anyone but the suggester and the pull request author(s).