Implement `echo' task action

lib/producer/core/actions/echo.rb

@@ -0,0 +1,11 @@
+module Producer
+  module Core
+    module Actions
+      class Echo < Action
+        def apply
+          env.output arguments.first
+        end
+      end
+    end
+  end
+end

lib/producer/core/task/dsl.rb

@@ -10,7 +10,8 @@ module Producer
           end
         end
 
-        define_action :sh, Actions::ShellCommand
+        define_action :echo,  Actions::Echo
+        define_action :sh,    Actions::ShellCommand
 
         attr_accessor :actions