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.
I'm currently hesitant to expose general functionality here. If that bar shows up at the top, I want it to be for a very good reason, to ensure people don't start to mentally block it out.

If we come up with a third reason to use this bar, we'll refactor this code to something more generic.