Add nav.json file for navigation config.
Separate out navigation into own template file.
Amend readme with navigation details.
Add nav entry for root path.
Add function to load and initialize navigation.

This function iterates through nav.json, reading the entries there
and setting one up to be highlighted if the current route deems it.

Also, rename loadAsset to loadAssets.
Change error message to use 'message' property.
Move navigation into own template file.