Adding US bank holiday

[andrewhawkins13]

I see where you define formal vs informal. We consider formal dates as government-defined holidays. These could be the kinds of holidays where everyone stays home from work or perhaps are bank holidays but it is not required for a holiday to have these features to be considered formal.

Can we add some way to know if a holiday is also a US bank holiday (regardless of formal vs informal)? It would be helpful to identify bank holidays recognized by the Federal Reserve - https://www.chicagofed.org/utilities/about-us/bank-holidays

[DagonWat]

@ppeble @andrewhawkins13 I think it would make sense to introduce us_fr as US federal reserve holidays, I will be willing to create a PR if this idea holds.