Use simple-redux-router UPDATE_PATH action type to clear counter

[naw]

This PR illustrates how to clear instance-specific state by listening to the UPDATE_PATH action type triggered by simple-redux-router. The basic idea is certain reducers that hold page-specific data (like insurance coverages) would pre-emptively reset themselves every time the route changes.

This PR is only addressing how to reset state, not how to fetch stale data or initial data.

The alternative to this approach is using onEnter to dispatch an action that clears the state for a specific reducer. This is the approach in the main routed-email-app branch.