event_command: add songpause and songresume events

[simpkins]

This adds code to invoke the event_command handler when playback is paused or resumed. This makes it possible to write an event_command handler that reports the current playback status.

[PromyLOPh]

As mentioned elsewhere (this is not the first time this feature has been proposed): These events are not enough to deduce the state of pianobar. They merely exist to support desktop notifications and limited scripting (like scrobbling to last.fm, if that’s still a thing). If you really need headless operation and full control, use pianod.