Use a custom quit message when gracefully shutting down the IRC bot

Summary: Fixes T3173. This doesn't actually fix T3173 but I'm going to redirect that. It does make the bot quit IRC gracefully, with a nicer quit message, which can be customized.

Test Plan: Got a bot to quit IRC with nice messages.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T3173

Differential Revision: https://secure.phabricator.com/D10257