Automatic resource tracking

[Mark Wainwright]

Hi.
Thanks for making this plugin - it's unbelievably useful.

For my game I've built quite an ergonomic wrapper around it and I think might be worth adding something like it to this crate.
Basically, you call app.init_tracked_resource<T>() for some Resource, and then that resource gets initialized with the value in the PkvStore and every time the resource changes, the store updates. You never need to use the PkvStore directly.

Happy to contribute code / examples / documentation if this is considered worth adding.