Add ability to join channels with a password

[vthunder]

A few irc channels require a password to join (admittedly it's rare), it would be nice to support that.

I took a quick peek at the source—I think this could be added via another JSON parameter in the channel purpose field, and then the context could be populated with another object mapping channels to passwords. Either that, or turn the channels array into an array of objects, each channel having a name plus other optional fields (like password).

Haven't dug into it beyond that, let me know if there's interest (and how you'd like it to work) and I can hack on it a bit over the weekend. I haven't checked if the underlying irc lib supports passwords, that would turn this into a bigger project.