playground: add ctrl-enter shortcut for gofmt (as in the tour)

LGTM=adg
R=adg
CC=golang-codereviews
https://golang.org/cl/120420044

godoc/static/playground.js

@@ -285,6 +285,9 @@ function PlaygroundOutput(el) {
           run();
           e.preventDefault();
           return false;
+        } if (e.ctrlKey) { // +control
+          fmt();
+          e.preventDefault();
         } else {
           autoindent(e.target);
         }

godoc/static/static.go

@@ -2307,6 +2307,9 @@ function PlaygroundOutput(el) {
           run();
           e.preventDefault();
           return false;
+        } if (e.ctrlKey) { // +control
+          fmt();
+          e.preventDefault();
         } else {
           autoindent(e.target);
         }