Added features to non multiple selects
Added features to non multiple selects

- added a single class to the selected-tag for non multiple selects
- added styling to the single selected-tag to differentiate it from the default 
rinterliche/vue-select,sagalbot/vue-select,rinterliche/vue-select,Vinelab/vue-select-checkbox,Vinelab/vue-select-checkbox,sagalbot/vue-select