There are third party tools like Graphite, CodeApprove, and Reviewable that will basically add these workflows on top of the existing GitHub API.

There's a much more detailed and complex answer to this question (TL;DR you can kind of fake it in various weird ways without too much effort) but that's probably the best approach if you're willing to fork over some money. Graphite in particular seems pretty well reviewed.

You may also want to look into Sapling and tools like git-branchless, which will change your workflow to better accommodate this kind of thinking.