Add href property to the router state object

Adding `href` to the router state object gives access to the
`$state.href` function in React code. This allows the creation
of urls for states, which is critical when a component links to
another part of the app and should be crawlable by a crawler.

Author: rufman

src/router-middleware.js

@@ -28,6 +28,7 @@ export default function routerMiddleware($state) {
           payload: {
             currentState: $state.current,
             currentParams: $state.params,
+            href: $state.href,
             prevState: getState().router.currentState,
             prevParams: getState().router.currentParams,
           },