Feature Request: Review single commit

Plannotator is amazing, thanks for making it.

I work with stacked diffs a lot. One thing I would love to have is the ability to review "commit-by-commit". Today, we have "Line of Work" as an option, and then you can choose the base. It would be awesome if, in the "Line of Work" mode, there was the option to select just one of the commits above your chosen base (e.g., master), and review that.

Let me know what you think...I'd be happy to send a PR.