[Feature Request] Set object default value using label for select

### Existing Component
Yes

### Component Name
el-select

### Description
I have an object that I want to set as default value for el-select.

There is a prop "value-key" that determines which field from the object to treat as a key. But it's also used as a label in this case.

How to set a "value-label" so the select's input displays custom label rather then the value of the key?

I use the below "hack" which is not really reliable:

this.$nextTick(() => {
let label = object[this.labelKey]
let elem = this.$refs.selectElem
elem.selectedLabel = label
})

What is the correct way to accomplish that?