Add aggreateId as part of the scenario

This was a bit hard to comment, so I PR-ed your PR :)

What do you think of this solution? 
In our opinion this makes it more explicit so your tests will be better readable.

So you would us it like this:

```php
$this->scenario
  ->withAggregateId($userId)
  ->given($eventsBefore)
  ->when($command)
  ->then($eventsAfter);
```
Doh, I forgot that one :)