title: Adding span next to input field with simple_form and custom wrapper
body: <p>I have custom wrapper:</p>
  
  <pre><code>  config.wrappers :input_group, tag: 'div', class: 'form-group', error_class: 'has-error' do |b|
      b.use :html5
      b.use :placeholder
      b.optional :maxlength
      b.optional :pattern
      b.optional :min_max
      b.optional :readonly
      b.use :label, class: 'control-label'
  
      b.use :input, class: 'form-control', wrap_with: { tag: 'div', class: 'input-group' }
      b.use :error, wrap_with: { tag: 'span', class: 'help-block' }
      b.use :hint,  wrap_with: { tag: 'p', class: 'help-block' }
    end
  </code></pre>
  
  <p>And a form with input:</p>
  
  <pre><code>&lt;%= simple_form_for @event do |f| %&gt;
    ...
    &lt;%= f.input :start_date, as: :string, wrapper: :input_group %&gt;
    ...
  &lt;% end %&gt;
  </code></pre>
  
  <p>It generates html:</p>
  
  <pre><code>&lt;div class="form-group string optional event_start_date"&gt;
    &lt;label class="string optional control-label" for="event_start_date"&gt;Start date&lt;/label&gt;
    &lt;div class="input-group"&gt;
      &lt;input class="string optional form-control" type="text" name="event[start_date]" id="event_start_date"&gt;
    &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>I'd like to add span, next to input element:</p>
  
  <pre><code>&lt;div class="form-group string optional event_start_date"&gt;
    &lt;label class="string optional control-label" for="event_start_date"&gt;Start date&lt;/label&gt;
    &lt;div class="input-group"&gt;
      &lt;input class="string optional form-control" type="text" name="event[start_date]" id="event_start_date"&gt;
  ----&gt; &lt;span class="input-group-addon"&gt;TEST&lt;/span&gt;&lt;----
    &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>How can I do this ? (span text will be different for each input element)</p>