Allow setting default classes on a per-form basis

Currently, default classes are set and read from the laravel-form-builder config file. This works fine most of the time, but I've come across several cases where I would like to change these for a particular form, but don't want to have to manually specify them for each form field. I propose adding another layer to pull default values from, in the form's $formOptions attribute. I think this would just be a matter of checking for a value first in FormField::allDefaults, and if found using that instead of the config file value. This way, you could do something like this:

class SomeForm extends Form
{
    protected $formOptions = [
        'defaults' => [
            'label_class'   => 'control-label',
            'wrapper_class' => 'col-sm-2',
        ],
    ];

    public function buildForm()
    {
        $this->add('inline-sub-form', 'form', [
            'class'        => $this->formBuilder->plain([
                'defaults' => [
                    'label_class'   => 'fancy-control-label',
                    'wrapper_class' => 'col-sm-11',
                ],
            ])->add('asdf', 'number'),
        ]);
    }
}

Thoughts? I thought I would throw this out there first, but I can do a PR.