Add instanceReadyCallback prop to allow attaching a custom instanceReady callback

[fernandomm]

This allows user to set a function that will be attached to CKEditor's instanceReady event.

It's useful because some events like fileUploadRequest can only be attached to the editor instance.

Here is an example:

<template>
  <div>
    <vue-ckeditor :instanceReadyCallback="ckEditorReadyCallback"></vue-ckeditor>
  </div>
</template>

<script>
export default {
  data() {
    return {
      ckEditorReadyCallback: function(readyEvent) {
        readyEvent.editor.on('fileUploadRequest', function(evt) {
          var xhr = evt.data.fileLoader.xhr

          xhr.setRequestHeader('My-Custom-Header', 'Example')
        }
      }
    }
  }
}
</script>