accept a callable function to `Mux::mount()` which can build a child mux

example usage:

```php
$mux = new Mux();
$mux->mount('/api', function(Mux $mux) {
    $mux->get('/endpoint', ['Controller', 'actionMethod']);
});
```

if you're okay with this addition I'll add a bunch of tests to cover it ( I just hate writing tests when the feature itself gets denied :P )