Default require path to features unless specified explicitly.

In general there seem to be some weird dependencies (or lack there of) between
configuration options and information in `Cucumber::Cli::Configuration#feature_files`
that could probably be clarified with a little refactoring.

One example that this feature exposed directly is that
`Cucumber::Cli::Configuration#feature_files` does not
depend on `Cucumber::Cli::Configuration#feature_dirs` though their names
certainly imply dependence.

I had deleted `Cucumber::Cli::Configuration#feature_dirs` to eliminate
the confusion, but added it back in when I realized it was used in
`features/docs/post_configuration_hook.feature`