演示地址：http://issues.wenzhixin.net.cn/bootstrap-table/methods/mergeCells.html
Merge some cells to one cell, the options contains following properties:
index: the row index.
field: the field name.
rowspan: the rowspan count to be merged.
colspan: the colspan count to be merged.
$table.bootstrapTable('mergeCells', {index: 1, field: 'name', colspan: 2, rowspan: 3});
功能描述：合并单元格