Have the ability to chose any executor in LoadDataFixturesDoctrineCommand

[BafS]

Using LoadDataFixturesDoctrineCommand it's possible to choose which purger we want to run (--purger) but that's unfortunately not the possible with the executor; ORMExecutor is hardcoded in the class (https://github.com/doctrine/DoctrineFixturesBundle/blob/3.4.x/Command/LoadDataFixturesDoctrineCommand.php#L154).

I have my own executor because of this issue (PR still open #209), I need to create a new command too, it would be great to have more flexibility.

And because fields are private I can't extends it, I basically need to copy/paste the command to change this line.

What do you think?