add user defined commands to config

[an-prata]

Allows for users to add commands that can be called by mpris-notifier in order to control other parts of their system. These commands are called for all signals, not just track changes.

This is especially useful for programs like waybar, where you can update a module based on a signal, and if you do so without an update rate it is much more recourse efficient.

[l1na-forever]

Hey Evan, looks great! Thanks so much for sending this :) 🎉💖

[l1na-forever]

Hey, I noticed that this ends up triggering pretty often from being in handle_signal ; I think this'd be more appropriately fired at the same time as a pending notification would be, in handle_pending. What do you think?

[an-prata]

I thought I did this before, but obviously not (probably placed it at the end of the function or something); just tested that and yeah, that should be better, I was under the impression that a signal would not be sent when a player becomes innactive, which is important in my case.

[an-prata]

Went ahead and did that in #8 for convinience.

[an-prata]

Oh, I completely missunderstood your suggestion, but that does make more sense. I will work on that soon.

[an-prata]

commands are called from within SignalHandler::handle_pending in #9