Allow multiple calls to `App.Router.map`

I've found a need to be able to add new routes at runtime (one of my apps allows plugins). Upon looking through docs and API, I couldn't find a way to do this so I went to source and still didn't find anything.

Attached is code that allows you to call `App.Router.map` multiple times without overwriting the previous behavior.

I've written two tests for it because the change is minor. I'm happy to write additional tests or receive guidance from the team.