title: Setting Vector Feature Fill Opacity when you have a hexadecimal color
body: <p>I'm trying to set the fill opacity of a vector feature in OL3 and can't figure out how to do it if I have a hexadecimal value...I've seen examples with rgba. Any suggestions?</p>
  
  <p>Here's what I have: </p>
  
  <pre><code>style : function(feature, resolution) {
            return [new ol.style.Style(
            {
              stroke : new ol.style.Stroke(
              {
                color : feature.get('color'),  
                width : 2
              }),
              fill : new ol.style.Fill(
              {
                color : feature.get('color'), //'#FF0000'
                opacity : 0.2   // I thought this would work, but it's not an option
              })
            })]
          }
  </code></pre>