state: channel: add Nicks() and NicksStr() accessors for nicks on channe...

Hi Alex,

I wanted to retrieve all known nicks of a channel (state.Channel struct) but lookup/nicks are private.
This pull request add Channel.Nicks() returning []*Nick and NicksStr() returning []string. The second one is just for convenience, similar to IsOn/IsOnStr.

Or maybe there is another way to get this already?

Cheers,
StalkR