title: simple_form bootstrap 4 radio buttons
body: <p>I am trying to set simple_form up with bootstrap 4 alpha, I am having trouble with the custom radio buttons. The html output I am trying to get to seems quite straight forward but I cant seem to get the configuration right.</p>
  
  <p>target output:</p>
  
  <pre><code>&lt;label class="col-xs-2 col-form-label"&gt;Label&lt;/label&gt;
  &lt;div class="col-xs-10"&gt;
    &lt;label class="custom-control custom-radio"&gt;
      &lt;input id="radio1" name="radio" type="radio" class="custom-control-input"&gt;
      &lt;span class="custom-control-indicator"&gt;&lt;/span&gt;
      &lt;span class="custom-control-description"&gt;Toggle this custom radio&lt;/span&gt;
    &lt;/label&gt;
    &lt;label class="custom-control custom-radio"&gt;
      &lt;input id="radio2" name="radio" type="radio" class="custom-control-input"&gt;
      &lt;span class="custom-control-indicator"&gt;&lt;/span&gt;
      &lt;span class="custom-control-description"&gt;Or toggle this other custom radio&lt;/span&gt;
    &lt;/label&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>Attempted config:</p>
  
  <pre><code>  config.wrappers :horizontal_radio, tag: 'div', class: 'form-group row', error_class: 'has-error' do |b|
      b.use :html5
      b.optional :readonly
  
      b.use :label, class: 'col-xs-2 col-form-label'
  
      b.wrapper tag: 'div', class: 'col-xs-10' do |ba|
        ba.wrapper tag: 'label', class: 'custom-control custom-radio' do |bb|
          bb.use :input, class: 'custom-control-input'
          bb.wrapper tag: 'span', class: 'custom-control-indicator' do
          end
          bb.use :label_text, class: 'custom-control-description'
        end
      end
    end
  </code></pre>
  
  <p>current output:</p>
  
  <pre><code>&lt;label class="col-xs-2 col-form-label radio_buttons optional"&gt;Label&lt;/label&gt;
  &lt;div class="col-xs-10"&gt;
    &lt;label class="custom-control custom-radio"&gt;
      &lt;input type="hidden" name="store[payment_type]" value=""&gt;
      &lt;span class="radio inline"&gt;
        &lt;input class="custom-control-input radio_buttons optional" type="radio" value="true" name="store[payment_type]" id="store_payment_type_true"&gt;
        &lt;label class="collection_radio_buttons" for="store_payment_type_true"&gt;Yes&lt;/label&gt;
      &lt;/span&gt;
      &lt;span class="radio inline"&gt;
        &lt;input class="custom-control-input radio_buttons optional" readonly="readonly" type="radio" value="false" name="store[payment_type]" id="store_payment_type_false"&gt;
        &lt;label class="collection_radio_buttons" for="store_payment_type_false"&gt;No&lt;/label&gt;
      &lt;/span&gt;
      &lt;span class="custom-control-indicator"&gt;&lt;/span&gt;
      Payment type
    &lt;/label&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>This is not too far off but it is not right and does not currently work. I cant seem to land on the right configuration to get the output correct. I have tried to use <code>wrap_with</code> but I need to add extra elements into the wrapper, and the wrapper class only seems to be the form_field wrapper not an item_wrapper.</p>
  
  <p>If someone could point me in the right direction that would be really useful.</p>