Add new App event "page", emitted when a new Page instance is created

[ericyhwang]

While the App's "ready" and "load" events provide the app's current Page instance when emitted, those events only fire a single time per App instance.

There are a couple scenarios where an App instance can create a new Page instance after initial load:

• A full client-side route change inside the app (non-transitional route)
• A code-initiated client page refresh with app.history.refresh()

That means an app can't rely on the "ready" and "load" events to do custom initialization on Page instances.

To allow such customizations, this adds a new "page" event, emitted each time an app creates new Page instances.

Note - this new event is emitted both during server-side rendering, as well as during client-side initialization and page changes.

[craigbeck]

👍 looks good