<index: attributes="accent">
  <div class="dmd-progress-container{{if loading}} dmd-progress-container--showing{{/if}}">
    <div class="dmd-progress mdl-progress{{if loading}} mdl-progress__indeterminate{{/if}}">
      <div style="width: 0%;{{if accent}}background-color:{{accent}}{{/if}}" class="progressbar bar bar1"></div>
      <div style="width: 100%;{{if accent}}background-color:{{accent}}{{/if}}" class="dmd-progress---bufferbar bar bar2"></div>
      <div style="width: 0%;{{if accent}}background-color:{{accent}}{{/if}}" class="auxbar bar bar3"></div>  
    </div>
  </div>

<index: attributes="accent class">
  <div class="dmd-progress-container{{if loading}} dmd-progress-container--showing{{/if}}">
    <div class="dmd-progress mdl-progress{{if loading}} mdl-progress__indeterminate{{/if}}{{if class}} {{class}}{{/if}}">
      <div style="width: 0%;{{if accent}}background-color:{{accent}}{{/if}}" class="progressbar bar bar1"></div>
      <div style="width: 100%;{{if accent}}background-color:{{accent}}{{/if}}" class="dmd-progress---bufferbar bar bar2"></div>
      <div style="width: 0%;{{if accent}}background-color:{{accent}}{{/if}}" class="auxbar bar bar3"></div>  
    </div>
  </div>

Add support for extra class
Add support for extra class
