Update Validator.php

[karol-may]

Adding Weighted Checksum Check i.e. for polish VAT ID

[dannyvankooten]

Hello @karol-may,

Nice work! Thank you for submitting as a PR. I have two questions though:

• Where can I find some documentation (by the Polish government, preferably) on this behavior? Do all Polish VAT number match this format?
• Can you add some tests for this, eg one for a VAT number without a weighed checksum (any non PL number as of now), one for a Polish VAT number that doesn't match and one valid one?

[dannyvankooten]

Closing because of inactivity, but in case you're keen to get merged just let me know and we can pick up where we left.