message: fix(hideSelected): allow in single selection
  closes
diff: @@ -82,7 +82,7 @@ export class ItemsList {
  }
  this._selectionModel.select(item, multiple, this._ngSelect.selectableGroupAsModel);
  - if (this._ngSelect.hideSelected && multiple) {
  + if (this._ngSelect.hideSelected) {
  this._hideSelected(item);
  }
  }
  