Expose API notifying about loaded CKEDITOR namespace

[jacekbogdanski]

Are you reporting a feature request or a bug?

In some use cases, we would like to access CKEDITOR namespace before initializing editor instances to customize it, e.g. when adding external plugins. I suppose it could be a callback created directly on the Vue CKEditor module, but I'm leaving implementation details to the developer.

E.g.

import { namespaceLoaded } from 'ckeditor4-vue';

namespaceLoaded( ( CKEDITOR ) => {
  CKEDITOR.addExternal( 'editorplaceholder', 'https://url-to-editor-placeholder/plugin.js' );
} ); 

[CKEditorBot]

Closed in #66

[jacekbogdanski]

We've decided to move namespace-loaded directive to namespaceloaded event. See #69 for more details.