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/dsl.rb

@@ -2,11 +2,23 @@ module Producer
   module Core
     class Task
       class DSL
+        class << self
+          def define_action(keyword, klass)
+            define_method(keyword) do |*args|
+              @actions << klass.new(@env, *args)
+            end
+          end
+        end
+
+        attr_accessor :actions
+
         def initialize(&block)
-          @block = block
+          @block    = block
+          @actions  = []
         end
 
         def evaluate(env)
+          @env = env
           instance_eval &@block
         rescue ConditionNotMetError
         rescue NameError => e