Added optgroups_with_options_for_select helper to form_options_helper for wrapping option tags in optgroups.

Hey, I'd like to get some feedback on this patch:
http://rails.lighthouseapp.com/projects/8994/tickets/977-patch-added-optgroups_with_options_for_select-helper-to-form_options_helper-for-wrapping-option-tags-in-optgroups

Thanks!