func Parse() string {
	selected := MustParse(CommandLine.Parse(os.Args[1:]))
	if selected == "" && CommandLine.cmdGroup.have() {
		Usage()
		CommandLine.terminate(0)
	}
	return selected
}
Parse and return the selected command. Will call the termination handler if
an error is encountered.
package kingpin
import (
	"os"
	"path/filepath"
)
var (
	CommandLine = New(filepath.Base(os.Args[0]), "")
)
func Command(name, help string) *CmdClause {
	return CommandLine.Command(name, help)
}
func Flag(name, help string) *FlagClause {
	return CommandLine.Flag(name, help)
}
func Arg(name, help string) *ArgClause {
	return CommandLine.Arg(name, help)
}