Add support for away-notify

This adds support for the [away-notify](https://github.com/ircv3/ircv3-specifications/blob/master/extensions/away-notify-3.1) client capability. Please do **not** merge this, there's still some work to do.

- [x] Add code to support away-notify
- [x] Test on a server with `away-notify` support
- [x] Write unit tests