Ability to add event-based messages or bot activations to rooms (ie. 'On Join' bot triggers)

[jordanhenderson]

Adding an ability to define room specific ‘on join’ messages to be automatically sent when a user joins a room. Could help handle initial prompts/informational messages, ie:

1. User joins room A
2. Room 'onJoin' message triggers installed bot (using a predefined message; displaying this initial message should be optional)
3. Bot responds with initial message (ie. information on how to use the bot), avoiding having to run an initial command for the room upon joining. Can assist with initial room setup.
4. This could also be paired with 'user specific' messages, to allow a message to be sent to a specific user rather than all users in the channel.