defaultPositioner = function(selectable) {
    return {
      'top': selectable.offsetTop,
      'left': selectable.offsetLeft,
      'width': selectable.offsetWidth,
      'height': selectable.offsetHeight
    };
  }
Default positioner. It computes selectable elements' positions, necessary
to identify selected elements as the selection box is dragged around.

This default implementation computes selectables' positions via offset
measurement. This works accurately if the element the plugin applies to
(the selection container) is their offset parent.

@param {!Element} selectable A selectable element, i.e. any element
    matching the plugin 'filter' option.
@return {!Object.<string, number>} An object containing the 'top', 'left',
    'width' and 'height' properties. 'top' and 'left' define the distance,
    in pixels, of the top-left corner of the selectable from the top-left
    corner of the selection container, accurate at the time the call is
    made. 'width' and 'height' define the outer width and height
    (including border, but not margin) of the selectable element.