code: 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;
  }
docs: 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}