Create IPC socket on UNIX platforms (#1018)

* 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

src/spotify.rs

@@ -32,7 +32,7 @@ use crate::ASYNC_RUNTIME;
 
 pub const VOLUME_PERCENT: u16 = ((u16::max_value() as f64) * 1.0 / 100.0) as u16;
 
-#[derive(Clone, Debug, PartialEq, Eq)]
+#[derive(Clone, Debug, PartialEq, Eq, Serialize)]
 pub enum PlayerEvent {
     Playing(SystemTime),
     Paused(Duration),