Expose module heirarchy via context

[mholt]

Would it be useful to expose the "path" or hierarchy to every module instance through the caddy.Context? In other words, a Context could return for an instance of a module the instance of a parent/host module that loaded it.

Right now, I think the only way to access a specific instance of a loaded module is to use the App() method which can only return an app module, and those are singletons.

I don't think we have a pressing need for this right now, but it's an interesting idea I had that I wanted to put in the issue tracker in case it becomes useful for something.

[mholt]

Actually this turned out to be useful for events, and it's been implemented in #4912.