wip: Add continent codes

[Bost]

I'm sending you a PR just to show some progress, so it's more of a Request For Comments than a PR.

Most important is the TODO at the end of the countrycodes.py. In addition to that I also should think about proper initialization of the continent_codes_list. Which one (of the following) should it be?

   continent_codes_list: list = None,
   continent_codes_list: list = [],
   continent_codes_list: list = continents.default_continent_codes_list,

And also I've noticed you require test cases and such... uuh, ooh. And maybe a bit of documentation (in the README.md or somewhere else) won't hurt neither... ooh, eeh.

[ExpDev07]

Looks good ! Just need to sort the test cases :)

[Bost]

Still just a work-in-progress (backing up my work)