Implement basic task actions API:

* Add Action base class;
* Implement Task::DSL.define_action(keyword, klass);
* Add Task::DSL#actions accessor;
* Apply task DSL actions when Task instances are evaluated.

lib/producer/core/task.rb

@@ -9,7 +9,9 @@ module Producer
       end
 
       def evaluate(env)
-        DSL.new(&@block).evaluate(env)
+        dsl = DSL.new(&@block)
+        dsl.evaluate(env)
+        dsl.actions.map(&:apply)
       end
     end
   end