Support multiple Beanstalk accounts in one Bot instnace

[okonet]

We don't support multiple accounts in the bot. And since there is no way to add several bots to one Slack team, we might want to add this.

[derekrushforth]

This would be awesome. A couple thoughts:

1. Add additional username and token fields to our bot.yml config. This is a bit hacky and we'd also have to see if Beep Boop supports optional fields. It would also be a bit messy because when someone adds our bot, the fields wouldn't be grouped in Beep Boop's UI.
2. A more optimal solution, yet doesn't seem supported, would be if Beep Boop had grouped arrays or something equivalent in their config. That way when someone adds our bot they could optionally "Add an additional BS account" or something. Might be worth submitting a feature request to them.