AngularJS bi-directional bindin between local property and parent scope property

drink.js

/*
 <div ng-app="drinkApp">
     <div ng-controller="AppCtrl">
     Ctrl
     <input type="text" ng-model="ctrlFlavor">
     Dir
     <div drink flavor="ctrlFlavor"></div>
     </div>
 </div>
*/

var app = angular.module("drinkApp", []);

app.controller("AppCtrl", function($scope) {
    $scope.ctrlFlavor = "blackberry";
})

app.directive("drink", function() {
    return {
        scope: {
            flavor: "="
        },
        template: '<input type="text" ng-model="flavor"> '
    }
})