Adding option for a default handler for RouterHandler

When I am developing a web app, I would usually like the main parts of the application to be loaded as-is. Except for some special circumstances where I would like specific routing (where I would plug in my back-end, etc.) 

Adding an option to add a defaultHandler to the RouterHandler. If a defaultHandler is specifies, and no router match is found, it will attempt to handle any other calls via the defaultHandler.