Add docs for extension {module, register()} variant

It'd be cool if the documentation mentions this variant of `extensions`:

```
extensions: {
".foo" {
module: "foo-loader", // load this module
register: (fooLoader) { // pass it to this function
fooLoader.register();
}
}
}
```

I see that it's already explained in [js-interpret's readme](https://github.com/js-cli/js-interpret#extensions) so even linking there, but specifically in relation to the above syntax, would be helpful for newcomers.