Add bootstrap 4 renderer (+ tests)?

Bootstrap 4 is in beta so IMHO it's safe to assume they are not going to change classes by a lot (which was the case in the alphas.. blegh).

Maybe I can see about making a bootstrap 4 renderer, if so I will add a pull request.

Migration instructions are here:

https://getbootstrap.com/docs/4.0/migration/#navs (for li / a)
https://getbootstrap.com/docs/4.0/migration/#navbar (for the containers and uls)