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(
                            'btn_append' => ['name' => 'search', 'type' => 'submit']
                        )
                    )
                ]);
```
Refactor

Renaming all btn references to button.
Merge pull request #287 from rdohms/input-group-buttons

Input Group Button Support