Add new feature differenceInBusinessDays

[codinsonn]

Sister PR to #1154

Should partly solve #584. Allthough it does not take holidays into consideration, it does exclude weekends and should have some value for end users.

All in all, it is just a thin wrapper around eachDayOfInterval, filtering out the weekends though.

Example usage:

var amountOfBusinessDays = differenceInBusinessDays(
    new Date(2018, 0, 1),
    new Date(2019, 0, 1)
)
// => 261 (52 weekends excluded = 104 days skipped)

[kossnocorp]

That sounds great, thank you! Please rebase with master so I can review the diff.

[kossnocorp]

For consistency, the first argument should be the later date: difference(4, 2) // => 2 and difference(2, 4) // => -2.

[kossnocorp]

I'll finish the PR.

[kossnocorp]

Please note that I've changed the function behavior:

• I've swapped the order of arguments, so the later date comes first. Now it's consistent with differenceInDays and differenceInCalendarDays
• For the sake of consistency, the function now also returns a negative number if the earlier date comes first.
• The function returns Invalid Date if any of the arguments is invalid.
• I subtract 1 from the length of the interval because it contains both starting and ending dates. The difference between two dates should not be 2

[kossnocorp]

It's done!

[kossnocorp]

This PR was released as v2.0.0-alpha.32: https://gist.github.com/kossnocorp/a307a464760b405bb78ef5020a4ab136#v200-alpha32

[ro-savage]

@kossnocorp - great work release this. We've been using a custom function with quite similar code in our code base. However, we also take into account 'holidays' by allowing an array of excluded dates to be passed.

Would you be interested in a PR that allows people to add in an array of dates (holidays) to be excluded from differenceInBusinessDays?