docs(runnableExamples): add "edit in Plunker" button

The "runnableExample.template.html" template overrides the one in the
dgeni-packages "examples" package with a similar template that also has
a link to a special Plunker URL that can pull in the example from our
code.angularjs.org website.

docs/app/src/docs.js

@@ -2,6 +2,8 @@ angular.module('DocsController', [])
 
 .controller('DocsController', function($scope, $rootScope, $location, $window, $cookies, NG_PAGES, NG_NAVIGATION, NG_VERSION) {
 
+  $scope.docsVersion = NG_VERSION.isSnapshot ? 'snapshot' : NG_VERSION.version;
+  
   $scope.fold = function(url) {
     if(url) {
       $scope.docs_fold = '/notes/' + url;