Allow blocking reviewers to be added via the CLI
Summary: Ref T10939. Fixes T4887. Supports "username!" to add a reviewer as blocking.
Test Plan: Added and removed blocking and non-blocking reviewers via CLI.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T4887, T10939
Differential Revision: https://secure.phabricator.com/D15934