title: Angular material multiselect dropdown with select all option
body: <p>I am using angular material multiselect with select all option.<br> Refer the stackblitz link below.<br>
  <a href="https://stackblitz.com/edit/angular-material-with-angular-v5-znfehg?file=app/app.component.html" rel="nofollow noreferrer">https://stackblitz.com/edit/angular-material-with-angular-v5-znfehg?file=app/app.component.html</a> <br>
  Currently whichever options I select appear in the textbox of multiselect which is desired behaviour. <br>
  I want to display only "All" text when I select all options, instead of showing each and every item In the list.<br>I tried several ways with template reference variable but it is not working. Kindly help on this.</p>