Add support for +#channel and @#channel messages

[dominikh]

Apparently there's an extension to the IRC protocol that allows sending messages to all voiced people or all ops in a channel, by addressing the message to +#channel and @#channel respectively.

Cinch should suport this, by extracting the actual channel name, and setting a flag indicating that the message was sent to a limited number of users.

Brought to my attention by leftylink on IRC, who saw it via caitlin/cinch-resistancegame#41

[dominikh]

9dc50ab adds support for receiving and responding to such messages. What's left to do is add new APIs to Channel for sending STATUSMSG messages..