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.

spec/producer/core/recipe_spec.rb

@@ -24,6 +24,10 @@ module Producer::Core
       it 'assigns nil as a default filepath' do
         expect(recipe.filepath).to be nil
       end
+
+      it 'has no task' do
+        expect(recipe.tasks).to be_empty
+      end
     end
 
     describe '#code' do
@@ -64,6 +68,13 @@ module Producer::Core
         expect(task).to receive(:evaluate).with(env)
         recipe.evaluate(env)
       end
+
+      it 'assigns the evaluated tasks' do
+        dsl = Recipe::DSL.new { task(:some_task) { } }
+        allow(Recipe::DSL).to receive(:new) { dsl }
+        recipe.evaluate(env)
+        expect(recipe.tasks.first.name).to eq :some_task
+      end
     end
   end
 end