表格最下面一行 建议加一个合计  

- [ ] I have searched the [issues](https://github.com/vueComponent/ant-design-vue/issues)         of this repository and believe that this is not a duplicate.
        
        
### What problem does this feature solve?
合计汇总

### What does the proposed API look like?
footer 属性添加后，footer 是在表格的滚动条外。 我们希望的合计是在表格数据的最后一行。  基于vue