Add Democratic Republic of the Congo holidays

[arkid15r]

Not to be confused with the Congo

Useful links:

• https://en.wikipedia.org/wiki/Public_holidays_in_the_Democratic_Republic_of_the_Congo
• https://www.timeanddate.com/holidays/dr-congo/

[PPsyrius]

@arkid15r How should we refer to this in the code? now that congo is already used by the Republic of Congo.

Would a democractic_republic_of_congo, democractic_republic_congo, or dr_congo works out better?

[arkid15r]

I guess it could be dr_congo for now. @KJhellico what's your opinion on this?

[KJhellico]

I think we can use dr_congo now, as v1 will have ISO 3166 codes anyway.