Add an `unsubscribe` subcommand to the pr and issue commands

As far as I know, there is currently no out-of-browser way to
unsubscribe from an issue or pull request. Typically, git-hub does not support this.

This functionality would really be helpful because it could then be
invoked directly from within an e-mail client.

Start with the existing `pr` and `issue` command entry points and inspect how they handle related GitHub operations. Determine the expected unsubscribe behavior for both resources, then verify that each command can perform it from the terminal and that the existing command tests cover the result.

Rédigé par le modèle d'indexation à partir du texte de l'issue.