title: Angular 2 get classnames as input from host and apply to child element
body: <p>What's the best way to get a string of class names in an Angular 2 component and add it to its child? I would also like to detect if the string is empty or if the option is not used and not add the class.</p>
  
  <p>My Component looks like this:</p>
  
  <pre><code>@Component({
      selector: "my-dropdown-toggle",
      template: `
          &lt;button &lt;Input Class from the _classname variable here&gt;&gt;
              &lt;ng-content&gt;&lt;/ng-content&gt;
              &lt;i class="fa fa-arrow"&gt;&lt;/i&gt;
          &lt;/button&gt;
      `,
      host: {
          "[class.dropdown-toggle]" : "true"
      }
  })
  export class MyDropdownToggle {
    @Input() _classname: string;
    ...
  }
  </code></pre>
  
  <p>And I would like to use it like this:</p>
  
  <pre><code>&lt;my-dropdown-toggle [_classname]="btn btn-danger"&gt;Hello&lt;/my-dropdown-toggle&gt;
  </code></pre>