Rate Limiting Feature Discussion

As pointed out in the feature suggestion issue, seamless rate limitation is important for the usability of the bot. I think it's a good idea to dedicate an issue to this feature in order to discuss an approach for implemetnation.

The rate limitation threshold depends on the Bot's user status in the channel:

- 20 per 30 seconds, if not mod / operator
- 100 per 30 seconds if mod / operator
- 50 per 30 seconds for known bots
- 7500 per 30 seconds for known bots

[Source](https://dev.twitch.tv/docs/irc#irc-command-and-message-limits)

This requires to introduce a new data structure that fulfills the following tasks:

- keep track of messages sent in a particular channel
- if rate limit is reached, buffer messages in a FIFO queue and send them whenever a message slot becomes available again
- keep track of the Bot's user level
- keep track of a reference to the say-function so it can decide on its own when to send buffered messages

A useful data structure for implementation is a [circular buffer](https://www.npmjs.com/package/circular-buffer).

In the future this data structure can also be used to save channel-specific information like current users.

Start with the Twitch IRC command and message limits linked in the issue, then review the per-channel tracking, FIFO buffering, user-level handling, and say-function coordination described here. Done means the bot applies the stated limits and sends buffered messages when capacity returns.

Written by the indexing model from the issue text.