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