code: public ArrayList<String> attrs(String cssSelector, String attrName) {
          ArrayList<String> result = new ArrayList<String>();
          Elements eles = select(cssSelector);
          for (Element ele : eles) {
              if (ele.hasAttr(attrName)) {
                  result.add(ele.attr(attrName));
              }
          }
          return result;
      }
docs: 获取网页中满足指定css选择器的所有元素的指定属性的集合
  例如通过attrs("img[src]","abs:src")可获取网页中所有图片的链接
  
  @param cssSelector
  @param attrName
  @return