app.Usage() ignores default command and doesn't recognize flags related to the default command.

``` go
package main
import (
"gopkg.in/alecthomas/kingpin.v2"
"os"
)
var (
app = kingpin.New("app", "A command line tool.")
old = app.Command("old", "").Hidden().Default()
verbose = old.Flag("verbose", "verbose mode").Hidden().Bool()
)
func main() {
kingpin.MustParse(app.Parse(os.Args[1:]))
app.Usage(os.Args[1:])
}
```
**Command**
```
go run app.go old --verbose
```
**Output**
```
usage: App old []

Flags:
--help Show context-sensitive help (also try --help-long and --help-man).
```

**Command**
```
go run app.go --verbose
```
**Output**
```
app: error: unknown long flag '--verbose'
exit status 1
```

### Expected behavior

`go run app.go --verbose` and `go run app.go old --verbose` should both display the same usage without any error as `old` is a default command.

> `app.parse` works fine as it doesn't ignore default command.