Implement `set?' task keyword:

When called from a task, will return true when given key exists and
false when key does not exist.

lib/producer/core/env.rb

@@ -2,7 +2,7 @@ module Producer
   module Core
     class Env
       extend Forwardable
-      def_delegators :@registry, :[]=
+      def_delegators :@registry, :[]=, :key?
 
       attr_reader   :input, :output, :error_output, :registry, :logger
       attr_accessor :target, :verbose, :debug, :dry_run