support table thead slot

[pipinet]

difficult to get two columns aligned with two table.

in source code 578 line

if (istable) {
        let tableElements = [];
        if (this.$scopedSlots['thead'] !== void 0) {
          tableElements.push(this.$scopedSlots['thead']());
        }
        tableElements.push(h('tbody', list));
        list = [h('table', tableElements)]
}

[molon]

Why not directly construct the entire container through a callback? table/colgroup/thead/tbody/blabla...

function callback(h, list) list {
     table/colgroup/thead/tbody/blabla...
}

Oh. May also need to feedback an extra heights.

[tangbc]

Table still has many questions, I plan in the future, but not soon, I want close this temporary.