Add a per-user rate limit for commands

[EverOddish]

We might want to create a TwitchUser object to store information about a specific user, rather than distribute it around the code base. Could also apply to WelcomeMessage described in issue #7

This would allow a rate-limit to be set per user, so that no single user can spam a lot of commands at once, without affecting the cooldown of other users that aren't spamming.