[WIP] Introducing batch actions

@dylanschoenmakers , what do you think about this? Two important things to note;

A `grid` has a new method `configureOptions`:

```php
<?php

namespace AppBundle\Datagrid\Grid;

use Opifer\CrudBundle\Datagrid\Cell\Type\TextCell;
use Opifer\CrudBundle\Datagrid\DatagridBuilderInterface;
use Opifer\CrudBundle\Datagrid\Grid\AbstractGrid;
use Symfony\Component\OptionsResolver\OptionsResolverInterface;

class UserGrid extends AbstractGrid
{
    /**
     * {@inheritDoc}
     */
    public function buildGrid(DatagridBuilderInterface $builder)
    {
        $builder->addColumn('name', new TextCell(), ['label' => 'User']);
    }

    /**
     * {@inheritDoc}
     */
    public function configureOptions(OptionsResolverInterface $resolver)
    {
        $resolver->setDefaults([
            'batch_actions' => [
                'approve' => 'admin_payouts_batch_process'
            ]
        ]);
    }
}

```

When the `batch_actions` option is set, the grid gets a checkbox before each row and a little dropdown with batch actions at the bottom of the grid.
The `batch_actions` receives an associative array, where the key is the action and the value the route.
The route can be either a string, like `admin_payouts_batch_process`, or an array like `['admin_payouts_batch_process', ['param' => $value]`.