Implement `ask' recipe keyword

lib/producer/core/prompter.rb

@@ -0,0 +1,21 @@
+module Producer
+  module Core
+    class Prompter
+      attr_reader :input, :output
+
+      def initialize(input, output)
+        @input  = input
+        @output = output
+      end
+
+      def prompt(question, choices)
+        cs = choices.each_with_index.inject('') do |m, (c, i)|
+          m += "#{i}: #{c}\n"
+        end
+        output.puts "#{question}\n#{cs}Choice:"
+        choice = input.gets
+        choices[choice.to_i]
+      end
+    end
+  end
+end