Update IRC appender to v3 appender style

Refers to: https://github.com/ptaoussanis/timbre/issues/41

Includes some small incidental fixes/tweaks, but maintained backwards compatibility I believe.

Non-backwards compatible changes that present themselves:

1. Replace global `conn` atom with one scoped/created inside `make-irc-appender`. 

2. Replace `prefix-fn` pattern with `fmt-output-fn` usage

Also happy to add some tests (guess I'd just mock the irclj functions?)