set default channel

[nminchow]

Allow admins to set a default channel. This adds a bit of complexity to the send interface. I see two options:

1. when a default is set, essentially make it the only allowed channel and make the send commands automatically send to it. This introduces a bit of complexity in how the help command works, and could potentially confuse people.
2. introduce a new set of commands that allow you to send to the default channel send_d(+r)