Add support for optgroups

[hobailey]

I appreciate this repo is no longer supported, but for the sake of those still using FancySelect and want to support styling for older browsers, this update allows for optgroups and their labels to be taken into account (previously, they were totally ignored). All feedback to the PR welcome :-)

[pmcnano]

@hobailey Thanks for this! 👍 I had to adjust it a bit, but worked perfectly for my needs!