Implement set' and get' recipe keywords:

`set' registers a value at given index in env registry, `get'
retrieves a value at given index.

lib/producer/core/recipe/dsl.rb

@@ -37,6 +37,14 @@ module Producer
             task("#{name}", *args, &block)
           end
         end
+
+        def set(key, value)
+          env[key] = value
+        end
+
+        def get(key)
+          env[key]
+        end
       end
     end
   end