title: woocommerce_form_field and hidden fields
body: <p>I'm trying to add custom fields to the WooCommerce checkout and there seems to be no output for hidden fields. </p>
  
  <p>In <code>woocommerce-template.php</code>, hidden fields fall into this switch case : </p>
  
  <pre><code>default :
      $field = apply_filters( 'woocommerce_form_field_' . $args['type'], '', $key, $args, $value  );
      break;
  }
  </code></pre>
  
  <p>How would I go about adding a <code>woocommerce_form_field_hidden</code> action which outputs a hidden field. I've tried multiple things which don't work. Ultimately, I'm not able to figure out how to pass the function parameters. </p>
  
  <pre><code>add_action('woocommerce_form_field_hidden', 'my_form_field_hidden');
  if ( ! function_exists('my_form_field_hidden') ) {
      function hp_form_field_hidden() {
  
          $field = '&lt;p class="form-row ' . implode( ' ', $args['class'] ) .'" id="' . $key . '_field"&gt;
              &lt;input type="hidden" class="input-hidden" name="' . $key . '" id="' . $key . '" placeholder="' . $args['placeholder'] . '" value="'. $value.'" /&gt;
              &lt;/p&gt;' . $after;
  
          return $field;
      }
  }
  </code></pre>
  
  <p>All the help is appreciated.</p>