[Feature Request] Custom value display when select

### Existing Component
Yes

### Component Name
Select

### Description
I wana custom input with phone number and select country code, example code below:
< el-input v-model="form.phone">
< el-select
slot="prepend"
v-model="form.prefixPhone"
>
< el-option
label="Viet nam ( 84)"
value=" 84"
>
< /el-option>
< el-option
label="Ameican ( 1)"
value=" 1"
>
< /el-option>
< /el-select>
< /el-input >
I expect it show 84 or 1 when selected and "Viet Nam ( 84)" "Ameican ( 1)" when show option to select