feat(Navbar): Added dark prop

[Lobosque]

If there is a light prop to add the navbar-light class, then to keep consistency there should also be a dark prop to add the navbar-dark class.
This PR adds the dark prop which will add the aforementioned class when set. With a test and the prop entry in the comments.

[TheSharpieOne]

This is already addressed in the v5 branch which accounts for changes in bootstrap 4 beta. A release will be published as soon as it is ready, we are waiting on a few more changes/fixes.
See #487