add CSS opacity property to config of Selection-js

selection-advanced-usage.html

<script src="selection.min.js"></script>
<script>
  var selection = new Selection();
  selection.config({
    facebook: true,
    twitter: true,
    search:true,
    copy:true,
    speak:true,
    backgroundColor: 'crimson',
    iconColor: '#fff',
    opacity: 0.8,
  }).init();
</script>