Detect existing features in subfolders

This is an alternative to https://github.com/codegram/spinach/pull/159. 

This patches the feature generator to check for an existing implementations of a feature file in subdirectories of steps e.g.

Given a feature at

    features/analytics/admin_analytics.feature

and an implementation at

    features/steps/analytics/admin_analytics.rb

when you run

    $ spinach --generate

then a new file will _not_ be generated at:

    features/steps/admin_analytics.rb