feat(routesCdnPath): Allow users to specify a custom local nav file

Closes #581

This provides a `.configURL` config parameter on `routesCdnPathProvider`, so you can run a local nav JSON file during development. 

This will be the preferred way for people to test changes to the official encore-ui-nav files. Do local changes using this, then send a PR against encore-ui-nav with the modified JSON.
Have you thought about maybe this being a function?

The only reason I can come up with as to why is if somehow the `customURL` property got changed post instantiation.  Which is pretty obscure considering that they would be swtiching navs? plus I guess that wouldn't work :grin:

Ignore me!