Spotcheck functionality

This release includes the possibility to spot-check a random subset of segments.

const validator = new DashValidator("http://example.com/example.mpd");

validator.load().then(() => {
  return validator.spotcheckSegments(null, 200);
}).then((result) => {
  console.log(result);
});

Above example will pick 200 random segments specified in example.mpd and perform a verification on each (using a default verify function in this case).