fix(Help): subcommand help messages requested via <cmd> help <sub> now correctly match <cmd> <sub> --help
fix(Help): subcommand help messages requested via <cmd> help <sub> now correctly match <cmd> <sub> --help

Prior to this fix, runnning `prog help subcmd` would produce:

```
subcmd

USGAE:
[...]
```

Notice lack of `prog-subcmd`. Whereas running `prog subcmd --help` produced:

```
prog-subcmd

USGAE:
[...]
```

These two forms now match the correct one (latter).

Closes #539

kbknapp/clap-rs,clap-rs/clap,kbknapp/clap-rs,clap-rs/clap,clap-rs/clap