Allow configuration of where segments are displayed

This commit adds initialization options to peaks, allowing you to specify whether segments should appear in the Zoom View, the Overview, or both.

The default behaviour is for segments to appear in both views, so no existing code should be affected by this addition.

The following code initialization object will tell peaks to only show segments in the Zoom View, and not in the Overview.

```
var options = {
  container: document.getElementById('first-waveform-visualiser-container'),
  mediaElement: document.querySelector('audio'),
  dataUri: {
    arraybuffer: '/test_data/sample.dat'
  },
  showSegmentsInZoomView: true,
  showSegmentsInOverview: false
};

var peaksInstance = Peaks.init(options);
```