Add support for returning a Promise in afterPageReady

This is useful in case you want a page ready callback make the other page ready callbacks wait for it. The client-side PDF export is such an use case:

• the print preview needs to be created after the page is ready (and it can take some time)
• the browser's print() function needs to be called after the print preview is ready

So it would be great is the page ready callback that creates the print preview can return a promise that forces the print callback wait for the preview to be ready.