zepto扩展prevAll 与nextAll 方法

$.fn.prevAll = function(selector){
 var prevEls = [];
 var el = this[0];
 if(!el) return $([]);
 while (el.previousElementSibling) {
        var prev = el.previousElementSibling;
        if (selector) {
          if($(prev).is(selector)) prevEls.push(prev);
        }
        else prevEls.push(prev);
        el = prev;
      }
      return $(prevEls);
};

$.fn.nextAll = function (selector) {
      var nextEls = [];
      var el = this[0];
      if (!el) return $([]);
      while (el.nextElementSibling) {
        var next = el.nextElementSibling;
        if (selector) {
          if($(next).is(selector)) nextEls.push(next);
        }
        else nextEls.push(next);
        el = next;
      }
      return $(nextEls);
    };