FIX: uses simpler patternj for custom href on extra nav items

[jjaffeux]

The main advantage of this solution is that it will be called on each re-render whereas the previous is not once href has been set.

Example API:

      api.addNavigationBarItem({
        name: "foo",
        displayName: "Foo",
        customHref: function(category, args) {
          const router = api.container.lookup("service:router");
          const queryParams = { bar: "1" };
          return router.urlFor(router.currentRouteName, category, {
            queryParams
          });
        }
      });

[discoursebot]

You've signed the CLA, jjaffeux. Thank you! This pull request is ready for review.

[SamSaffron]

Cool, I say we try this out!