Add multiple channel support

Created by: ca1ek

You can now join multiple channels at once.

Channel names are stored in channels vec. Current channel's number is stored in current_channel.

/join pushes the channel name onto channels, and bumps channel number to the latest one(channels.len())

/next bumps channel number by one and prints the current channel, /back lowers it by one and prints.

Messages are displayed in #channel user: message format, later each channel could get it's own buffer and be displayed on channel switch.

Also, now /part works for parting, and /exit works for exiting.