Allow users to override default help behavior.

[ianlewis]

• CommandGroup is now public.
• Commander and CommandGroup name is accessible by Name()
• Commander, CommandGroup, and Command explations are now printed by overridable functions set on the Commander.
• Subcommands are visitable by the VisitCommands() method.
• Command groups are visitable by the VisitGroups() method.
• Important flags are visitable by the VisitAllImportant() method.
• Top level flags are visitable by the VisitAll() method.