title: How can I get optgroup label when select an option in vuejs?
body: <p>I want to get country group name when select option. I've countries with several group like </p>
  
  <pre><code>&lt;template&gt;
      &lt;select id="countries" v-model="country" @change="getCountryGroup($event)"&gt;
          &lt;optgroup label="Asia"&gt;
              &lt;option value="AF"&gt;Afghanistan&lt;/option&gt;
              &lt;option value="AM"&gt;Armenia&lt;/option&gt;
              &lt;option value="AZ"&gt;Azerbaijan&lt;/option&gt;
              ...
              &lt;option value="BD"&gt;Bangaldesh&lt;/option&gt;
              ...
          &lt;/optgroup&gt;
          &lt;optgroup label="Australia / Oceania"&gt;
              &lt;option value="AS"&gt;American Samoa&lt;/option&gt;
              &lt;option value="AU"&gt;Australia&lt;/option&gt;
              &lt;option value="CK"&gt;Cook Islands&lt;/option&gt;
          &lt;/optgroup&gt;
          &lt;optgroup label="Africa"&gt;
              &lt;option value="DZ"&gt;Algeria&lt;/option&gt;
              &lt;option value="AM"&gt;Angola&lt;/option&gt;
              &lt;option value="AZ"&gt;Azerbaijan&lt;/option&gt;
          &lt;/optgroup&gt;
          &lt;optgroup label="South America"&gt;
              &lt;option value="AR"&gt;Argentina&lt;/option&gt;
              &lt;option value="BO"&gt;Bolivia&lt;/option&gt;
              &lt;option value="BR"&gt;Brazil&lt;/option&gt;
          &lt;/optgroup&gt;
          &lt;optgroup label="North America"&gt;
              &lt;option value="US"&gt;United States&lt;/option&gt;
              &lt;option value="UM"&gt;United States Minor Outlying Islands&lt;/option&gt;
              &lt;option value="CA"&gt;Canada&lt;/option&gt;
          &lt;/optgroup&gt;
          &lt;optgroup label="Europe"&gt;
              &lt;option value="UK"&gt;United Kingdom&lt;/option&gt;
              &lt;option value="AL"&gt;Albania&lt;/option&gt;
              &lt;option value="AD"&gt;Andorra&lt;/option&gt;
          &lt;/optgroup&gt;
      &lt;/select&gt;
  &lt;/template&gt;
  </code></pre>
  
  <p>Suppose when I select <code>Bangladesh</code> then I want to get optgroup name <code>Asia</code>. Here is my vueJs method</p>
  
  <pre><code>getCountryGroup: function(event){
      console.log( event.target.getAttribute('label') );
  }
  </code></pre>