Implement ISO4217 currency validation rule

[bilfeldt]

This PR adds a new validation rule for validating ISO4217 currencies using their most common form which is a three letter code (examples: USD, EUR, CNY).

• Implement validation rule Currency()
• Add tests
• Add README.md description

This implementation intentionally uses the alcohol/iso4217 package for ISO4217 checks (the more downloaded package Payum/iso4217 is a fork of this one and has been abandoned).

Note that an alternative could be to rely on the already required package league/iso3166 which includes currency codes (see src/ISO3166.php@L2431), but I was not able to confirm if this list is in fact complete.

UPDATE: has been changed to league/iso3166

[freekmurze]

Thank you for this. Let's improve it a bit so people not using this rule, won't have to pull in an extra package.

[freekmurze]

Thanks!