title: Bootstrap / ngx-bootstrap - drop down mutiple checkbox option
body: <p>On our current Angular 5 project, we use <code>bootstrap 3.3.7</code> and <code>ngx-bootstrap 2.0.3</code></p>
  
  <p>I have been asked to implement a dropdown multi-select option as shown below. It will allow user to select multiple options from dropdown list.</p>
  
  <p><a href="https://i.stack.imgur.com/t1dE9.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/t1dE9.png" alt="enter image description here"></a></p>
  
  <p>How can I implement this feature?</p>