Translate fields using square brackets like those with periods

See #267

For example, the following inputs will both be translated with the same (Laravel) translation rule:

    Former::text('address[city]')
    Former::text('address.city')

    [ 'validation' => [
        'attributes' => [
          'address' => [
            'city' => 'City'
          ]
        ]
      ]
    ]