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.
Merge pull request #1 from felixclack/group-by-scenario

Option to split cucumber features by scenario.