wrap loadPlugin() in instanceReady event

I think `loadPlugin()` should be wrapped in `instanceReady` event.

Let's consider this situation.

Assumming ...
- our application has many page using  CKEditor.
- each pages have multiple instance of CKEditor.
- we want to add autosave plugin to **ALL** CKEditor instance without making change to all affected files.
- `config.autosave_SaveKey` should be something like this ... `autosave_[url]_[instanceName]`

In this case, we can use `instanceReady` event to modify `autoSaveKey` value to be unique in each instances.

```javascript
CKEDITOR.on('instanceReady', function(e) {
    e.editor.config.autosave_SaveKey = 'autosave_' + window.location + '_' + e.editor.name;
});
```
But problem is, we cannot use `instanceReady` to do the job since `loadPlugin()` is immediately called  in plugin's `init` callback, which make it execute before the `instanceReady` event.

With my pull request and this work around we can dynamically create a unique key for each instances without making change to all affected files. (Just place above code right after ckeditor.js <script> tag)

Thanks for the great plugin,
Porawit Poboonma