More formatting fixes

This pull request (Re)-fixes #9 by implementing almost all of the [Slack text formatting spec](https://api.slack.com/docs/formatting). When receiving messages we'll convert Slack's `<!everyone>`, `<#C1234>` and `<@U1234>` formatting into the equivalent text before passing it onto the rest of hubot.


Good idea! I did this in 426f16d4571113a33db7d058ca3d58711ea5d203