Add Array#insert

function insert(index) {
  var args = slice.call(arguments, 1);
  this.length = Math.max(this.length, index);
  index = index < 0 ? this.length : index;

  if (args.length > 1) splice.apply(this, [index, 0].concat(args));
  else splice.call(index, 0, args[0]);

  return this;
}