Add callback to stream that will be called a multiple of bufferlengths before the stream finishes.

[duysqubix]

When using a queue or sequence like in the beep examples it is possible to (dynamically) add streams without pausing playback (gapless). Would it be possible to get notified right before the end of a stream so an new one can be added to the queue without experiencing any pauses? So, a notification 1 or more bufferlengths away from the end of a stream so we have enough time to add a new one?

Original issue: faiface/beep#53