make everything private
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.