Simple menu example

Here's a simple navigation menu, based on the Pure site, of why it might be useful to use `express-map` when building your site.  You can take a look at the tutorial here:

https://github.com/clarle/express-map/tree/menu-example/examples/navigation-menu

The one thing that I'm not sure of is that I didn't get into a discussion about `pathTo`, and providing a context for a route.  I'm debating whether I should extend the example to encompass that or to keep it simple, at least for now.