Delegation of sub-commands

It would be great if I could add subcommands to `task`, e.g. if I call `task foobar` and there is an executable `task-foobar` in `$PATH`, then execute that instead. This is how Git and many other tools work, and it's a great way to extend functionality.

I thought this could maybe be done using the following configuration

```
alias.foobar=execute task-foobar
```

and there would be merit to requiring such explicit configuration, but it doesn't actually work, possibly due to #2105.