title: Symfony 4, add a text input field inside a ChoiceType
body: <p>I want to create a select box with three options of color, and in the third option I would like the user to input whichever he/she likes. How do I add a text input field inside the ChoiceType? </p>
  
  <p>The code looks something like this now: </p>
  
  <pre><code>public function buildForm(FormBuilderInterface $builder, array $options) {
      $builder-&gt;add('shifts', FormType\ChoiceType::class, [
          'choices' =&gt; [
              'black' =&gt; 0,
              'white' =&gt; 1,
              'customized' =&gt; 2,
          ],
          'multiple' =&gt; true,
          'expanded' =&gt; true,
          'label' =&gt; 'entity.user.shift',
      ]);
  }
  </code></pre>