Fix 969 add get selected option class

[sagalbot]

It's currently really difficult to add a custom class to the selected-option-container slot. That slot needs to be refactored, and this prop provides a lightweight solution to custom classes for selected options in the meantime.

Edit: Nov 18, 2019

Maybe this should just be getOptionClass, with a selected param provided to the callback. This way the same prop can be used for both situations.

---

Related #895
Closes #969

[coveralls]

Coverage increased (+0.02%) to 95.814% when pulling 5c9f898 on fix-969-add-get-selected-option-class into 1aef52a on master.

[McPo]

Running into this issue now, Is there any update to this draft feature?

My specific use case is are few of the options have an error and I would like to make those options red.

----- Update -----

Actually it wasnt as verbose as I was expecting re-defining the majority of the selected-option-controller

        <template #selected-option-container="{ option, deselect }">
          <div :class="{ 'vs__selected': true, 'error': errors.includes(option.node.id) }">
            {{ option.node.displayName }}
            <button type="button" class="vs__deselect" @click="deselect(option)" >
              <deselect></deselect>
            </button>
          </div>
        </template>
....
    components: {
      vSelect: vSelect,
      Deselect: vSelect.components.Deselect
    }