sometimes you want to know the 5th business day of the month. sometimes there are holidays. hmm.
working days = no weekends and no holidays -- didn't want to get confused with date-fns business day features.
you need to pass in your own holidays. see example-holidays.json
list out all the holidays between two dates
list all working days (no weekends or holidays)
add some number of working days to a date
is the day a working day
get the x working day starting at a day -- like get me the 5th working day of the month