Possibility to override default country and locale

[gogl92]

In some cases we may need to change the country to use like:

Holiday::country('us')->get($carbon)

and the locale

Holiday::country('Spain')->locale('es')

But still being able to work with the default country/locale if needed.

So we can do:

Holiday::country($user->country_code)->get($carbon)

And get the holidays based on the user's country code.

[gogl92]

I can issue a PR after the PHPUnit one 👍