pb-notify: Notify for all mirrored notifications and pushes

[tsani]

This should actually be configurable, so that you can choose exactly which notifications you would like to display. I'm thinking of a system of "notification policies" which are computations that determine whether a notification should be shown or not for a given event received in the stream. We can build an HTTP webservice into pb-notify so that policies can be changed dynamically, to (temporarily) silence certain types of notifications for instance.

[tsani]

Remark: if I implement this feature, then because of znc-push, this gives me a universal way to receive IRC notifications on all my devices.