Trigger input and keyup event on any content changes

MediumEditor users need to save editable HTML in some form input on every changes. So MediumEditor docs recommends to bind `input` event:

```js
editable.on('input', function () {
   var value = editor.serialize()['element-0'].value;
   input.val(value);
});
```

I add `input` and `keyup` events on any HTML changes, that I found in plugin.