Add a tracking mode

Hi, Daijiro Wachi.

This is a funny library, I play with it, but I notice it's hard to find the current note in the music.
So I added a tracking mode to track this current note for easier knowing where it is.

use it like this
```javascript
intro = beeplay({bpm: bpm, track: function(notes, trackId, stack) {
  console.log(notes);
}})
```