Implement Worker class:

* Move recipe processing code in the worker;
* Refactor CLI and use the the worker;
* Implement Recipe#tasks and remove tasks application during evaluation,
  tasks are now applied by the worker after all evaluations are done.

lib/producer/core/worker.rb

@@ -0,0 +1,13 @@
+module Producer
+  module Core
+    class Worker
+      def process(tasks)
+        tasks.each { |t| process_task t }
+      end
+
+      def process_task(task)
+        task.actions.each(&:apply)
+      end
+    end
+  end
+end