Support Postal Code Validation

[insightindustry]

Continuing with the localization themes outlined for milestone v.1.6.0, it may be helpful to add support for postal code (zip code) validation:

• validators.postal_code(value, ..., country = None) which validates value as a properly-formatted postal code within country (or checks against all countries if None)
• checkers.is_postal_code(value, country = None) which validates value against validators.postal_code()