#100 Add option to set a welcome message per channel

Currently the welcome message is hard-coded. It would be good to have the option to customise the message on a per-channel basis. Use case: a group has a forum (or other means of communication besides email) that they want to inform newcomers about, in the event they are temporarily unavailable to respond in the channel. I discussed the feature briefly with fr33domlover, and it might look something like this: - Have 3 config items: - `$message` (example name) - `$contact` (renamed from `$email` to take other values besides email) - `$folks` (existing node) - `$contact` and `$folks` can be inserted into `$message` - The message can be set via command, e.g. `!set channels.[#channel].message [welcome message]` Thanks!