support sub command

[humboldt-xie]

like docker
docker [OPTIONS] COMMAND

ex:
docker --debug run --it busybox

go:

type MyApp struct {
      Debug bool `flag:"debug"`
      CurCommand interface{} `flag:"&command"`
      Run  RunCmd `command:"run"`
      Build BuildCmd `command:"build"`
} 
type RunCmd struct {
       Interactive bool `flag:"i" help:"Keep STDIN open even if not attached"`
}