Implemented ban reason support into /ban.

Ticket: https://bukkit.atlassian.net/browse/BUKKIT-4285 (not mine)
This is a completely different way of implementing ban reason support, so incase https://github.com/Bukkit/Bukkit/pull/935 won't get merged I've made this one.
I apologize if I'm not doing this correctly, this is my first pull request.