Add -n short option for --next-failure

Created by: ianks

--next-failures is such a terrific feature! It really helps me to stay focused and write specs which are concise, and more easily squash bugs when they arise.

The only thing that I do not like about the feature is all of the typing involved when writing --next-failures. This PR makes a short option -n, to increase the ergonomics of using the feature. Less typing, happier hands!