title: mat-prefix to work on the input not the label
body: <p>So using material we have some code to ask a question, the input we would like to add a $ sign to the front of or use a place holder. inside of this mat field we also have a mat-label. If I use matprefix it puts the dollar sign infront of the label not the input field. Can anyone suggest a way to do this?</p>
  
  <pre><code>&lt;mat-form-field floatLabel="always" class="fb-form-field fb-auto-detail-question-group"&gt;
          &lt;!-- &lt;mat-label class="fb-question-label"&gt;Amount of service or tow? &lt;/mat-label&gt; --&gt;
          &lt;label class="sub"&gt;What was the amount of the service or tow bill?&lt;span class="frm__label--required"&gt;*&lt;/span&gt;&lt;/label&gt;
          &lt;input matInput formControlName="serviceBillAmount" class="fb-form-auto-input"  /&gt;
          &lt;span matPrefix&gt;$&amp;nbsp;&lt;/span&gt;
          &lt;mat-error *ngIf="fieldInvalid('serviceBillAmount')"&gt; The dollar amount you entered is invalid. Please try again.&lt;/mat-error&gt;
      &lt;/mat-form-field&gt;
  </code></pre>