support sub command 

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"`
}
```