For instance for when multiple ncspot instances are running. In that case the
PID will be appended to the path, i.e. `~/.cache/ncspot/ncspot.20707.sock`.
Also ncspot will now delete the socket on shutdown.
Fixes#1158
* Create IPC socket on UNIX platforms
Creates an IPC socket which remote programs/scripts can connect to. This
can be used to control ncspot or fetch the current playback status.
At the moment, only remote control is implemented. Next step is to send
the current player status as a JSON object.
Fixes#524
* Publish status changes to connected sockets
Whenever the playback mode (playing, paused, stopped) or the track changes, all
socket listeners will be notified.
Fixes#924, fixes#1019
* Document IPC feature
