Fix addons with Bootstrap 4

Usage with Laravel:
{{-- Basic input addon --}}
{!! Former::open()->method('GET') !!}
    {!! Former::text('test2')->append('€') !!}
{!! Former::close() !!}

{{-- Multiple input addons --}}
{!! Former::open()->method('GET') !!}
    {!! Former::text('test3')->append(['€', 'second']) !!}
{!! Former::close() !!}

{{-- Button addon --}}
{!! Former::open()->method('GET') !!}
{!! Former::text('test4')->append('<button class="btn">OK</button>')
!!}
{!! Former::close() !!}

{{-- Button addons --}}
{!! Former::open()->method('GET') !!}
{!! Former::text('test5')->append(['<button
class="btn">OK</button>', '<button class="btn">2</button>']) !!}
{!! Former::close() !!}

Credits: @mrextreme

Reference:
formers#581 (comment)

src/Former/Form/Group.php

@@ -469,7 +469,7 @@ protected function placeAround($items, $place)
 	{
 		// Iterate over the items and place them where they should
 		foreach ((array) $items as $item) {
-			$item             = $this->app['former.framework']->placeAround($item);
+			$item             = $this->app['former.framework']->placeAround($item, $place);
 			$this->{$place}[] = $item;
 		}
 	}

src/Former/Framework/TwitterBootstrap4.php

@@ -405,17 +405,25 @@ public function createPlainTextField(Field $field)
 	 *
 	 * @return Element A wrapped item
 	 */
-	public function placeAround($item)
+	public function placeAround($item, $place = null)
 	{
 		// Render object
 		if (is_object($item) and method_exists($item, '__toString')) {
 			$item = $item->__toString();
 		}
 
-		// Get class to use
-		$class = (strpos($item, '<button') !== false) ? 'btn' : 'addon';
+		$items = (array) $item;
+		$element = '';
+		foreach ($items as $item) {
+			$hasHtmlTag = strpos(ltrim($item), '<') === 0;
 
-		return Element::create('span', $item)->addClass('input-group-'.$class);
+			// Get class to use
+			$class = $hasHtmlTag ? '' : 'input-group-text';
+
+			$element .= $hasHtmlTag ? $item : Element::create('span', $item)->addClass($class);
+		}
+
+		return Element::create('div', $element)->addClass('input-group-'.$place);
 	}
 
 	/**