Add custom rake task to run cucumber features in parallel.

The official one provided by parallel_test gem requires some optional
parameter which makes the rake task uncomfortable to use.

lib/tasks/parallel_testing.rake

@@ -94,4 +94,9 @@ namespace :parallel do
   task :spec_legacy do
     sh "bundle exec parallel_test --type rspec -o '-I spec_legacy' spec_legacy"
   end
+
+  desc 'Run cucumber features in parallel (custom task)'
+  task :cucumber do
+    sh "bundle exec parallel_test --type cucumber -o '-r features' features"
+  end
 end