title: How to do checkAll options for angular2 dropdown multiselect
body: <p>I am using this library for multiselect options ,i have checkbox overthere, need to select all the checkbox while user click select all check box ,how to do this,any one have  implemented before.
  <a href="https://github.com/softsimon/angular-2-dropdown-multiselect" rel="nofollow noreferrer">https://github.com/softsimon/angular-2-dropdown-multiselect</a></p>