add MultiMessageOnJoinTrigger
add MultiMessageOnJoinTrigger

- backed by a flat json file, messages can be changed dynamically
- Hardcoded admins can allow other users to set entry messages for a groupchat
- All messages must be set in the groupchat, does not respond to private messages.

Efreak/node-steam-chat-bot,Efreak/node-steam-chat-bot,Steam-Chat-Bot/node-steam-chat-bot,Steam-Chat-Bot/node-steam-chat-bot