Add speaker.PlayAndWait

[MarkKremer]

Adds a quality of life function to play a sound and wait until it is finished. This function waits until the samples have reached the driver. Actual applications should probably not use this but it's useful for testing and example code.

// before
done := make(chan struct{})
speaker.Play(beep.Seq(streamer, beep.Callback(func() {
	done <- struct{}{}
})))
<-done

// after
speaker.PlayAndWait(streamer)

Downside: not having this function is a good reason to explain beep.Seq and beep.Callback. Now part of the tutorial needs to be rewritten.