display(target) {
    if (this.displaying) {
      this.displaying.resolve();
    }

    return this.q.enqueue(this._display, target);
  }
Display a point in the book
The request will be added to the rendering Queue,
so it will wait until book is opened, rendering started
and all other rendering tasks have finished to be called.
@param  {string} target Url or EpubCFI
@return {Promise}