feat(example:todomvc): add editing support

- implement "onfocusout" handler
- set autofocus
- handle enter, escape and tab on a todo item
- make the `<pre>` element between todo list items of margin 0 to be unnoticable

examples/assets/todomvc.css

@@ -1,5 +1,5 @@
 html,
-body {
+body, pre {
     margin: 0;
     padding: 0;
 }