Create GitHub Issues from Slack

If you spot a bug when you're on the go or discussing on Slack, don't bother booting up the GitHub interface; just write "BUG: Hovered link is invisible" into Slack and we'll take care of it for you

This sketch will listen in all Slack channels that @bots is in (don't forget to /invite @bots!) for messages beginning with BUG:, ISSUE: or TASK: and will open a issue on GitHub with a link back to the Slack chat.