title: Setting a default value on MatButtonToggleGroup directive
body: <p>In this material 2 snippet, how do we make sure that the button with the <code>draft</code> value happens to be the default one? <a href="https://material.angular.io/components/button-toggle/api" rel="nofollow noreferrer">Docs</a> mention the <code>selected</code> and <code>checked</code> directives, but they do not seem to work. </p>
  
  <pre><code>&lt;mat-button-toggle-group #group="matButtonToggleGroup" matInput name="status" [(ngModel)]="status" #field_status="ngModel"&gt;
      &lt;mat-button-toggle checked? selected? default? value="draft"&gt;
      DRAFT &lt;--how do we set this value to be the default selected/checked one? --&gt;
      &lt;/mat-button-toggle&gt;
      &lt;mat-button-toggle value="publish"&gt;
      PUBLISH
      &lt;/mat-button-toggle&gt;
  &lt;/mat-button-toggle-group&gt;
  </code></pre>