Type hinting in Command's constructor (value object mapping)

It would be useful to use a value objects in command, so if you type hint property, mapper will instantiate and pass object.

```
class RegisterCommand {

    /**
     * @var EmailAddress
     */
    private $email;
    
    private $password;

    public function __construct(EmailAddress $email, $password)
    {
        $this->email = $email;
        $this->password = $password;
    }
}
```