no way to add a hook to do something after parsing config

there should be a way to add a hook to perform an action after running the configuration parse stage (and after it passes).  something like:

```
moon.AfterParse(func() {
  // ...
})
```

and you should be able to register a bunch of stuff like that.  That way, in different files, you can add post-config hooks in `init()` instead of having a really cumbersome and huge `main()`.
