Allow toby to drop doorbell messages in customizable channels

@heymosef gave me an idea for a little augmentation of our doorbell script [`scripts/sms-doorbell.coffee`](https://github.com/CivicTechTO/hubot-toby/blob/master/scripts/sms-doorbell.coffee) that might he helpful in situations like this:


Would be cool if people could use a hubot command to rig up toby doorbell in arbitrary room. Or perhaps the better pattern would be to "borrow" the doorbell for specific periods of time. Should only take maybe an hour of work, and seems aligned with the larger goals of helping us do more things collaboratively (even just manning the door :) )

Example interaction:

```txt
## CHAT LOG for #our-one-day-hackathon

>>> janegoodall: /invite @toby #our-one-day-hackathon
*** @toby was invite into #our-one-day-hackathon by @janegoodall
>>> janegoodall: @toby doorbell borrow 6 hours
>>> toby: Sounds good! Any messages to our doorbell (780-652-2649 or 780-6LAB-6IX)
will be dropped in here until 5:30pm!
(The doorbell has been silenced in the default channel #organizing-open.)

## CHAT LOG for #organizing-open

>>> toby: The doorbell has been borrowed by #our-one-day-hackathon!
(It will only ring in that room until 5:30pm.)
```

Comments, feedback, and thoughtful critique is totes appreciated :heart:

Start with scripts/sms-doorbell.coffee and inspect how the existing doorbell messages are handled. Use the example interaction to clarify the command, borrowing period, target channel, and default-channel notification behavior. Done means messages route to the borrowed channel for the requested period and the default channel reports the change.

Written by the indexing model from the issue text.