Subscribe to specific IRC messages

[DrFrankenstein]

Who: Library users
What: Create observables that emit on specific IRC messages
Why: So that we don't always have to subscribe to messages() | filter([](m) { return m.command == "COMMAND"; }). Running those filters on every message can become heavy quickly (O(n); n: amount of subscribers).
Points: 5

This is not on a milestone yet, as performance impact hasn't yet been asserted.

Acceptance Criteria

• I can subscribe to something like session.messagesOfType("PRIVMSG").
• That observable would emit whenever a PRIVMSG is received.
• Those observables are kept track of somewhere and get purged when no one's subscribed to a message anymore
• Support for both commands and numerics.