Ability to become a reviewer that must accept before Pull request can be merged

Pull requests can be hard to manage at the moment where other reviewers may approve a request while you are in the middle of a review and then the whole thing can be merged.

I want the ability to be able to block the pull request until I have approved the request (or anyone can say they want to block it till they have approved it )

Currently you can kind of hack it by creating a task which will block the PR till it's ticked.

I would imagine I'd just click a button called "Reviewing" and have a rule "Don't allow merge with active reviewers" or some such.