优化 `layer.closeLast()` 方法，给第二个参数增加回调

[#I8JHBY](https://gitee.com/layui/layui/issues/I8JHBY)

src/modules/layer.js

@@ -1288,9 +1288,9 @@ layer.closeAll = function(type, callback){
 };
 
 // 根据弹层类型关闭最近打开的层
-layer.closeLast = function(type){
+layer.closeLast = function(type, callback){
   type = type || 'page';
-  layer.close($('.layui-layer-'+ type +':last').attr("times"));
+  layer.close($('.layui-layer-'+ type +':last').attr("times"), callback);
 };