Adding --fast-fail and --no-fast-fail

I'm kind of dyslexic and kind of forgetful.  After weeks of accidentally typing `rspec --fast-fail`, I've decided to just open a pull request.

Hopefully I'm not the only one who would appreciate this feature.  I'm not sure if I did the `.feature` file correctly, and not sure if you'd want me to add this to the `changelog` myself.  I also just copy/pasted the spec since a shared_examples block seemed overkill.

Thanks~