[Feature Request] 表格组件底部增加自定义行列（Add custom rows and columns at the bottom of the table component）

### Existing Component
是

### Component Name
el-table

### Description
el-table 组件列表底部可以插入任意自定义行列，作为表格数据总览统计，例如合计，但是不是每个单元格都有合计结果与其对应，可以合并所有单元格并且显示的列数只有两列，第一列显示“合计”标题,第二列显示数量，改行长度和表格宽度一样，可随着表格宽度伸缩，固定操作列等均可正常显示
（At the bottom of the el table component list, any custom row or column can be inserted as a summary of table data statistics, such as totals. However, not every cell has a total result corresponding to it. All cells can be merged and only two columns are displayed. The first column displays the "total" title, the second column displays the quantity, and the length of the changed row is the same as the table width. It can be expanded and expanded with the table width, and fixed operation columns can be displayed normally）