Logic for creating several tracks in TrackManager

[ilyamore88]

Add methods for working with several tracks in TrackManager

[neSpecc]

How it should look like (from user point):

let chill = new ChilloutAudio();

chill.addTrack({
  melody: 'A4 E4 D4',
  interval: 5,
  instrument: 'horn'
})

chill.addTrack({
  melody: 'A5 A5 A5 D4 D4',
  interval: 2,
  instrument: 'sine-wave'
})

chill.play();