Allows Seeder execution absent of a Command object

I might like to execute a Seed file outside the context of an artisan command. Eg:

```php
$seeder = App::make('My\Seeder\Class');
$seeder->run();
```

At the moment this fails because the seeder requires a command object for sending some output.

```
Call to a member function getOutput() on a non-object
```

This PR makes the command object optional.