Allow a setup-block for Observable#wait_for

[Burgestrand]

This block would be executed after all the event handlers have been hooked. It’d be useful for doing things where Hallon needs to know of events to work properly (see Playlist#upload in #95 for example).

This would allow an API similar to this:

playlist.upload do
  playlist.insert(track)
end

[Burgestrand]

The internals are changing too much with my experiments and I’ve noticed this issue is no longer applicable.