auto-invoke optgroups in CGI::FormBuilder

For option lists with three-item entries (value, label,
category), turn on the optgroups option in CGI::FormBuilder
for that field.
Documentation (user and code) tweaks

Just some clarifications, improved pod, etc about the
optgroups feature and behavior.
Merge pull request #33 from msouth/feature/optgroups

Feature/optgroups