[Feature Request] el-tree data both support multi-level and single-level array.

### Existing Component
Yes

### Component Name
el-tree

### Description
##### Now data (multi-level)
```javascript
let treeData = [
{
label: 'level-1',
children: [
{
label: 'level-2',
children: [ ... ]
}
]
}
]
```

##### Maybe Better data (single-level)
```javascript
let treeData = [
{
label: 'label-1',
parentLabel: null
},
{
label: 'label-2',
parentLabel: 'label-1'
},
{
label: 'label-3',
parentLabel: 'label-2'
}
]
```

##### Why we need single-level data?
Single-level data useful for F2E state management (Vuex, Redux or Flux) and B2E SQL normal response data.

单层数组对前端状态管理（Vuex，Redux 和 Flux）和后端默认 SQL 返回值更加友好。