title: ng-multiselect-dropdown with option group
body: <p>I am trying to group my options in ng-multiselect-dropdown. Is there any way, I cannot find any reference. Please see my code as below.
  <strong>.html</strong></p>
  
  <pre><code>&lt;ng-multiselect-dropdown [placeholder]="'Select one'" [data]="mutiData" [settings]="multiDataSettings"&gt;
  &lt;/ng-multiselect-dropdown&gt;
  </code></pre>
  
  <p><strong>.ts</strong></p>
  
  <pre><code>this.multiData = [
      { id: '1', value: 'A one', 'group':'A' },
      { id: '2', value: 'A two', 'group':'A' },
      { id: '3', value: 'B one', 'group':'B' },
  ];
  
  this.multiDataSettings = {
      singleSelection: true,
      idField: 'id',
      textField: 'value',
      itemsShowLimit: 1,
      groupBy: "group",
  };
  </code></pre>