Expose regions

Good day!

I was looking for a way to validate user supplied holiday regions against what was available in the `Holidays` class and, as such, needed a simple way to access the `@@regions` class variable (i.e rather than resorting to `Holidays.send(:instance_variable_get, :@@regions)`).  Please let me know if you would like me to make any additional changes to get this pull request accepted; I would really like to see it in upstream.

Thanks!