Add redis-backed chat history with setup in config

[scollett]

I implemented a history with Redis. It uses Redis lists with the chatroom id as the key. It keeps unlimited history in Redis, but only pulls from Redis the last 5 chat lines unless a higher/lower 'viewlimit' is set in the config file. The history is not saved in Redis or displayed unless there is a history entry in the config file.

[qdsang]

Great！

[LoicMahieu]

+1 Cool feature!