Support for ->required() on the label within a form group

By intercepting the `required($conditional)` function here we will add automatically the `control-label-required` CSS class to the `<label>` HTML element.

This way you are able to use CSS to style the `<label>` HTML element however you want, you can even use pseudo-elements to add an asterisk:
```css
label.control-label-required:after {
    content: " *";
}
```

Obviously we wont interfere with Form Builder and we'll call the `required()` function defined within `adamwathan/form` no matter what. The CSS class will just be added conditionally.

src/AdamWathan/BootForms/Elements/GroupWrapper.php

@@ -58,6 +58,16 @@ public function hideLabel()
         $this->labelClass('sr-only');
         return $this;
     }
+
+    public function required($conditional = true)
+    {
+        if ($conditional) {
+            $this->formGroup->label()->addClass('control-label-required');
+        }
+
+        call_user_func_array([$this->target, 'required'], [$conditional]);
+        return $this;
+    }
 
     public function inline()
     {