support table thead slot

difficult to get two columns aligned with two table.

in source code 578 line

```js
if (istable) {
        let tableElements = [];
        if (this.$scopedSlots['thead'] !== void 0) {
          tableElements.push(this.$scopedSlots['thead']());
        }
        tableElements.push(h('tbody', list));
        list = [h('table', tableElements)]
}
```