Add Env#output with $stdout as default

lib/producer/core/env.rb

@@ -2,12 +2,18 @@ module Producer
   module Core
     class Env
       attr_reader   :current_recipe
+      attr_writer   :output
       attr_accessor :target
 
       def initialize(recipe = nil)
         @current_recipe = recipe
+        @output         = $stdout
         @target         = nil
       end
+
+      def output(str)
+        @output.puts str
+      end
     end
   end
 end