Support for event_command

[nlowe]

On Linux and macOS I use event_command to scrobble to Last.FM. However, on the windows fork, the implementation is commented out:

https://github.com/thedmd/pianobar-windows/blob/191128aa504548a05a886a4b0a32e6f9bc8e299d/src/ui.c#L721-L828

[thedmd]

@nlowe

When porting indeed commented out that part due to use of fork() and at a time not clear understanding how I could implement that.

I will look into that next time I get back to pianobar.

[jonmchan]

+1 for this feature. I was looking into combining this with https://github.com/stuartleeks/wsl-notify-send such that you can have a notification of what the next song in the notification center.