Move namespaceLoaded directive into events

[jacekbogdanski]

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When working on docs for #67 I find out that we probably misplaced our namespaceLoaded directive with the event. It seems more semantic to treat it as an event since it's asynchronous and it's called only in a specific environment. It would be good to move this directive into events instead, so instead of writing:

<ckeditor :namespace-loaded="callback" />

we would have

<ckeditor @namespaceloaded="callback" />

(I've changed the name of the event on purpose as it's probably more convenient to write events without - character, and the camel case is discouraged by Vue).

For more details about the feature see #66

[CKEditorBot]

Closed in #70