Add support for using manual cooldowns on single commands

[jamesbt365]

Not sure if adding fields to Command is breaking, it can be constructed publicly so maybe? will rebase to next on request.

Currently it is treated like an override, but if allowing users to have manual cooldowns on for every command but a few select ones is desired, this could be an Option<bool>.

I have not adjusted the examples, but in the current state there isn't really anything to add. Maybe a comment somewhere saying that it can be done for specific commands?