Added the ability to use different weekend days.

[SaharZehavi]

for fri-sat weekends use.

use example:
bizniz.setWeekendDays([5,6]).addWeekDays(new Date(), 5) // will consider friday as the weekend and not sunday.

[jamesplease]

Thanks @SaharZehavi ! This looks promising.

Two things:

1. I'm not a fan of the "global" weekend setting. If we want an API like bizniz.setWeekendDays([5,6]).addWeekDays(new Date(), 5), it should use a Constructor and return an entirely separate bizniz. That way, you could set the weekend days, and then later do:

bizniz.addWeekDays()

and it would use the default weekends. Does that make sense?

Also, would you like to try your hand at adding unit tests?

[SaharZehavi]

that was what made sense to me.
if i would have any more time to work on it, i'll change it/add tests to it. can't really commit to it.

[jamesplease]

Sure thing. I wish I had time to help out, but time is seeming scarce these days for me!

Future visitors: if you want to see this get merged, feel free to jump in and update the API / write some tests. You can just make a PR into this branch.