Support ISO 3166-2 codes for subdivisions of countries

[mborsetti]

python-holidays only supports prov and state subdivisions, however states are subdivided into one or more from these categories:

• cantons
• commune
• councils
• counties
• departments
• dependencies
• districts
• divisions
• emirates
• entities
• governorates
• governorates
• islands, groups of islands
• Länder
• administrations
• municipalities
• parishes
• prefectures
• provinces
• rayons
• regions
• sectors
• states
• territories
• towns
• unitary authorities
• voivodships
• wards

Instead of expanding the types of subdivisions naming supported by this library, I propose adopting ISO 3166-2 (https://en.wikipedia.org/wiki/ISO_3166-2) which already standardizes all types of subdivisions into a non-overlapping namespace.

I envision being able to initialize holidays for the state of California in the USA (ISO 3166-2 code US-CA) as follows:
us_ca_holidays = holidays.US('CA'),
for the Emirate of Dubai as follows:
dubai_holidays = holidays.AE('DU'),
for the Ağsu rayon in Azerbajan as follows:
az-agu_holidays = holidays.AZ('AGU')
and so on.

@dr-prodigy I can work on a pull request [with backward compatibility] if I know it's going to be utilized, but won't work on it otherwise.

[dr-prodigy]

Hey there @mborsetti
sorry this proposal went unnoticed until now, my bad!

This is definitely a great idea, which would provide a definitely higher flexibility in the config/init phase.. especially if coupled to .CountryHoliday('[CODE]').

As long as we provide compatibility with previous versions, I totally agree with your analysis and would really appreciate your improvement. In the meanwhile, I can help you out with the task, if you wish.

thx loads 👍

[dr-prodigy]

Released with #608 merge. Thx!