Replace env.output interface with an IO like object

lib/producer/core/env.rb

@@ -1,18 +1,13 @@
 module Producer
   module Core
     class Env
-      attr_writer   :output
-      attr_accessor :target
+      attr_accessor :output, :target
 
-      def initialize
-        @output = $stdout
+      def initialize(output: $stdout)
+        @output = output
         @target = nil
       end
 
-      def output(str)
-        @output.puts str
-      end
-
       def remote
         @remote ||= Remote.new(target)
       end