Input Group Button Support

Added a new Extension to allow people to append buttons to inputs as well as
icons. Will add examples to the docs repo also. But this is what it looks
like:

```
$builder
           ->add('keywords', 'text', [
                   'label' => false,
                   'attr' => array(
                       'input_group' => array(
                           'button_append' => ['name' => 'search', 'type' =>
'submit']
                       )
                   )
               ]);
```

This closes #250.