function arrayInsert(array, items, index) {
    var startIndex = typeof index === numberType ? index : -1;
    if (startIndex < 0) startIndex = array.length - startIndex + 1;

    array.splice.apply(array, tempArray.concat(startIndex, 0, items));
    tempArray.length = 0;
  }
Insert an item or an array of items to array to a specified index. Mutates
the array. The index can be negative in which case the items will be added
to the end of the array.

@param {Array} array
@param {*} items
@param {Number} [index=-1]