Switching command based on os.Args[0]

It's happened a few times now that I've migrated separate utility programs in as sub-commands of a larger program that uses Kong. In those cases, it would sometimes be neat to continue to provide the old binary name (via a symlink, hard link, or whatever) and have things continue to work.

As a concrete example, there is currently `syncthing` (a program) and `strelaysrv` (a different, supporting program). Migrating `strelaysrv` into `syncthing` so that it can be called as `syncthing relaysrv ...` is easy and clean. It would be nice if we could also say something like

```
struct CLI {
Serve serveCmd `cmd:"" default:""`
// ...
Relaysrv relaysrv.CLI `cmd:"" default-when-argv0-is:"strelaysrv"`
}
```

and then calling the binary as `strelaysrv` would be equivalent to typing `syncthing relaysrv`.

Alternatively, there's perhaps some magic that can be done via variable interpolation to tweak what is the default command before calling `kong.Parse()`?

As it is, I'm setting up an args slice myself based on os.Args, tweaked to simulate that the longer form command was run, and then pass that to a kong parser.