Allows to generate multiple bundles once

When installing Sonata e-commerce, we have to run all those commands:

```bash
php app/console sonata:easy-extends:generate SonataBasketBundle
php app/console sonata:easy-extends:generate SonataCustomerBundle
php app/console sonata:easy-extends:generate SonataDeliveryBundle
php app/console sonata:easy-extends:generate SonataInvoiceBundle
php app/console sonata:easy-extends:generate SonataMediaBundle
php app/console sonata:easy-extends:generate SonataOrderBundle
php app/console sonata:easy-extends:generate SonataPaymentBundle
php app/console sonata:easy-extends:generate SonataProductBundle
```

I have updated the `sonata:easy-extends:generate` command to allow generating several bundles with only one command call:

```bash
php app/console sonata:easy-extends:generate SonataBasketBundle SonataCustomerBundle SonataDeliveryBundle SonataInvoiceBundle SonataMediaBundle SonataOrderBundle SonataPaymentBundle SonataProductBundle
```

This is not a BC break because this command can also be called as before with only 1 bundle name.