Method for closing the context and hence releasing the device

[duysqubix]

The audio device never releases after calling speaker.Init()
I suggest to extract the following code from Init():

if player != nil {
	done <- struct{}{}
	player.Close()
	context.Close()
}

to a separated public method.

What do you think?

BTW, great work! This pkg is the cornerstone of a project I'm working on

Original issue: faiface/beep#56

[punkscience]

I think I have also run into this issue. I am playing a list of songs in my playlist and it only successfully plays a single song to finish. On the second and every subsequent song, I get the following issue:

failed to initialize speaker: oto: context is already created

I attempted to defer close of the speaker and also to force close the speaker but it did not fix the problem.

[MarkKremer]

Continuation of the last comment in #143.