Allow passing holidays with each invocation.

[mstate]

Modified code, test and documentation to allow passing a holiday or array of holidays with each invocation. This is particularly useful when dealing with potentially different regional holidays during each invocations.

[bokmann]

Wow... I like this solution to a problem I've had (calculating backwards after snow days have happened), and it was less impactful that I would have considered.

It has a merge conflict with another pull I just merged, but if you resolve that it'll go into the next version!

Thanks for the first-time commit! Don't forget to add yourself as a contributor to the readme!

[RST-J]

We recently had a problem with a local holiday in our region that does not exist in other regions of Germany. It would be solvable with BusinessTime::Config.with but we were not aware of that method and the solution implemented here is less verbose.

Inspired by this I thought of an direct integration of the holidays gem into business_time. But I am not sure whether you would be up for it @bokmann? Reading the Not-Todo section and the following one about stability and change suggest rather not. But I thought I'd ask first before considering doing it in a fork.

[Laykou]

Do you think this could be merged, please?

[rmm5t]

Do you think this could be merged, please?

@Laykou This PR has merge conflicts that would need to be resolved first.

[Laykou]

@mstate Would you be willing to resolve the conflicts, please?

[cadenforrest]

I took a stab at resolving the conflicts in #217