Adding "--seed <seed>" parameter for seeding randomizer. Also prints seed.

Adds seed parameter for overriding the seed for the randomizer. 
This resolves issue https://github.com/fiunchinho/phpunit-randomizer/issues/1.

I've also added a custom ResultPrinter that outputs the seed from the run (custom or not), so a user can re-run with the same seed as previously randomized.

Would've loved to add some unit tests for this, but I decided to postpone that for now.