subcommands.go
func (cdr *Commander) ImportantFlag(name string) {
	cdr.important = append(cdr.important, name)
}
func (cdr *Commander) ImportantFlag(name string) {
	cdr.important = append(cdr.important, name)
}
// ImportantFlag marks a top-level flag as important, which means it
// will be printed out as part of the output of an ordinary "help"
// subcommand.  (All flags, important or not, are printed by the
// "flags" subcommand.)