Implement a key/value registry at env level

lib/producer/core/env.rb

@@ -1,18 +1,27 @@
 module Producer
   module Core
     class Env
-      attr_reader   :input, :output
+      attr_reader   :input, :output, :registry
       attr_accessor :target
 
-      def initialize(input: $stdin, output: $stdout)
-        @input  = input
-        @output = output
-        @target = nil
+      def initialize(input: $stdin, output: $stdout, registry: {})
+        @input    = input
+        @output   = output
+        @target   = nil
+        @registry = registry
       end
 
       def remote
         @remote ||= Remote.new(target)
       end
+
+      def [](key)
+        @registry[key]
+      end
+
+      def []=(key, value)
+        @registry[key] = value
+      end
     end
   end
 end