Signaling player state changes / finishing instead of polling player.IsPlaying()

[vsariola]

The examples show how to wait for player being finished as

for player.IsPlaying() {
  time.Sleep(time.Millisecond)
}

This smells badly. What is needed is players to signal something like a EOF, probably through a channel, so that waiting for the player to be finished could be done with something like

<-playerFinished

Alternatively, a sync.WaitGroup might be appropriate, with 1 counter. Player could have method Wait() just calling the Wait() method of the sync.WaitGroup. Then any number of goroutines / threads could wait for the player to be finished.