Pivot Table: allow empty columns

[dongnengyu]

透视表生成方式是否可以参考下python的numpy库，我在项目中不需要列标签，但是这个工具库不允许不设置列标签，一旦列标签设置为空就会报错。

下面是numpy的透视表生成方式

[xuri]

Hi @dongnengyu, thanks for your issue. I have added empty columns support for creating a pivot table. Please try to upgrade the library with the master branch code.