Merge pull request #222 from felixclack/use-binstubs-in-runner

Use binstubs in runner
indent
Thanks Felix!
v0.13.3
Remove duplicate private keyword
Mutation of groups not necessary.

As the return value of the method is the result of the call to `.map`,
it seems unnecessary to use the bang method.
Option to split cucumber features by scenario.

Inspired by [cukeforker](http://github.com/jarib/cukeforker). Sometimes
the time it takes to run a feature can vary even when containing the
same number of scenarios. In this case it might be more efficient to
group by scenario instead of feature.