Leverage express-state's `cache` option to improve expose() performance

This updates how Express YUI exposes data to the client by using app-scoped exposing and caching the result to increase the performance of serialization.

A non-enumerable `__modified__` flag has been added to the YUI `config` object which is used by the methods that modify the `config` to signal that it seems to be re-exposed and re-serialized by express-state.

Instead of using the `__modified__` flag, a different approach could be taken to leave it in the hands of the developer to re-expose the YUI config data if they change it.

**Note:** This PR depends on a pending Express State change which fixes an issue that arose in the Express YUI example apps: https://github.com/yahoo/express-state/pull/20

### Todos

- [ ] Evaluate whether we want an alternative approach to `__modified__`
- [ ] Merge https://github.com/yahoo/express-state/pull/20 and release `express-state@v1.2.0`
- [ ] Document changes in README and HISTORY