Added some unit tests

I renamed vimeoUrlToId to parseVimeoId and added more validations to check against invalid URLs, not sure if this is going to cause problems. I did it because there are not releases yet, also, I'm not sure if that method needs to be public. 
Please also check the constructor arguments in the method getVimeoFormatter in the test, since I am still learning Drupal,  I'm not sure if they make sense (it's working but please review them).
Also review the method getItems since I'm not sure if that's the correct implementation that should be passed to the method viewElements, it works but I'm not sure if it makes sense.
Is there a reason not to version composer.lock? :-)