Allow different messages per channel

[patcon]

Re-ticketed from #3

Allow a custom message for different rooms that might be entered. We don't have a need for this yet (that I know of), but I can imagine it being helpful down the road.

[patcon]

Possible approach:

welcome_messages
├── C04TJ34KQ.txt
├── general.txt
└── organizing-open.bot-testing.txt

Allow channel to be set via directory of files named after channel ID (regex: [0-9A-Z]+) or channel names (regex: [0-9a-z-_]+). dots (not valid in channel names) could delimit each room.