Added the Confirm built-in middleware

This middleware asks the user a question and expects a Y/N answer. This
middleware can be used with the Call middleware.

lib/vagrant/action.rb

@@ -16,6 +16,7 @@ module Vagrant
     # and are thus available to all plugins as a "standard library" of sorts.
     module Builtin
       autoload :Call, "vagrant/action/builtin/call"
+      autoload :Confirm, "vagrant/action/builtin/confirm"
     end
 
     module Env

lib/vagrant/action/builtin/confirm.rb

@@ -0,0 +1,28 @@
+module Vagrant
+  module Action
+    module Builtin
+      # This class asks the user to confirm some sort of question with
+      # a "Y/N" question. The only parameter is the text to ask the user.
+      # The result is placed in `env[:result]` so that it can be used
+      # with the {Call} class.
+      class Confirm
+        # For documentation, read the description of the {Confirm} class.
+        #
+        # @param [String] message The message to ask the user.
+        def initialize(app, env, message)
+          @app      = app
+          @message  = message
+        end
+
+        def call(env)
+          # Ask the user the message and store the result
+          choice = nil
+          choice = env[:ui].ask(@message)
+          env[:result] = choice && choice.upcase == "Y"
+
+          @app.call(env)
+        end
+      end
+    end
+  end
+end

test/unit/vagrant/action/builtin/confirm_test.rb

@@ -0,0 +1,21 @@
+require File.expand_path("../../../../base", __FILE__)
+
+describe Vagrant::Action::Builtin::Confirm do
+  let(:app) { lambda { |env| } }
+  let(:env) { { :ui => double("ui") } }
+  let(:message) { "foo" }
+
+  ["y", "Y"].each do |valid|
+    it "should set the result to true if '#{valid}' is given" do
+      env[:ui].should_receive(:ask).with(message).and_return(valid)
+      described_class.new(app, env, message).call(env)
+      env[:result].should be
+    end
+  end
+
+  it "should set result to false if anything else is given" do
+    env[:ui].should_receive(:ask).with(message).and_return("nope")
+    described_class.new(app, env, message).call(env)
+    env[:result].should_not be
+  end
+end