Add greece holidays

[lowv-developer]

Contributing a new country ?

• Have you checked to ensure there aren't other open Pull Requests for the same country?

1. Create a new class in the Countries directory. It should extend the Country class.
2. Add a test for the new country in the tests directory.
3. Run the tests so a snapshot gets created.
4. Verify the result in the newly created snapshot is correct.

[Nielsvanpach]

Thanks!