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 thought about that too. The trick is that `customURL` can *only* be accessed/changed in a `.config` block, it's not available anywhere else. So whatever value it has at instantiation is the value it will have forever.