Add captcha whitelist made with redis

[comendantmc]

Feel free to add it to your code if you want. Let me know if you need some adjustments to this PR

[Galster-dev]

Hello!

1. What's the point of using redis for this?
2. Whitelists should be used with both IP and nickname
3. Isn't it already implemented here ?

[comendantmc]

1. I might have quite an unusual use case where an api adds players to the whitelist automatically and redis seemed like a good idea for least overhead realtime checking without needing to pool changes periodically or putting commands to remote console. Maybe it can be useful to other people as well as I read somewhere you're adding redis dependency anyway.
2. I see the potential security risk here, but it only adds the whitelist for the captcha and not the other checks. It can probably be improved by adding rate limit to connections with the same nickname or/and remembering previous ip addresses of the user.
3. As I said the already implemented one whitelists users from all the checks, but this one just disables the captcha

[Galster-dev]

Just to point out: I'm not a contributor of this project.
In my opinion "unusual" use cases should not modify the plugin that much. A good solution would be to make an API to allow extension plugins force skip some checks maybe?

@hevav