Add one or more classnames to an element
@param {Element} element
@param {Array.<string>|String} array of classnames or string with
classnames separated by whitespace
@return {Element}
function addClass(element, className) {
  var classNames = className;

  function _addClass(el, cn) {
    if (!el.className) {
      el.className = cn;
    } else if (!hasClass(el, cn)) {
      if (el.classList) {
        el.classList.add(cn);
      } else {
        el.className += ' ' + cn;
      }
    }
  }

  if (!isArray(className)) {
    classNames = className.trim().split(/\s+/);
  }
  classNames.forEach(_addClass.bind(null, element));

  return element;
}