Support multiple Navs without explicit IDs

[chrisvxd]

We use portals, and set a div with ID nav-root. If you're using more than 1 nav on the page, you'll need to have two different IDs. You are able to explicitly set the nav-root by adding the navRootId prop:

<Nav navRootId="my-id" />

However:

1. This is undocumented
2. This should happen automatically, if possible. It's also important for us to support SSR, and make sure the ID persists between server and client.