Set cover and create cover document if needed.

        :Args:
          - file_name: file name of the cover page
          - content: Content for the cover image
          - create_page: Should cover page be defined. Defined as bool value (optional). Default value is True.

def set_cover(self, file_name, content, create_page=True):
        """
        Set cover and create cover document if needed.

        :Args:
          - file_name: file name of the cover page
          - content: Content for the cover image
          - create_page: Should cover page be defined. Defined as bool value (optional). Default value is True.
        """

        # as it is now, it can only be called once
        c0 = EpubCover(file_name=file_name)
        c0.content = content
        self.add_item(c0)

        if create_page:
            c1 = EpubCoverHtml(image_name=file_name)
            self.add_item(c1)

        self.add_metadata(None, 'meta', '', OrderedDict([('name', 'cover'), ('content', 'cover-img')]))