Wraps the output HTML with a list of all available ``Keyword``
        instances that can be clicked on to toggle a keyword.

def format_output(self, rendered_widgets):
        """
        Wraps the output HTML with a list of all available ``Keyword``
        instances that can be clicked on to toggle a keyword.
        """
        rendered = super(KeywordsWidget, self).format_output(rendered_widgets)
        links = ""
        for keyword in Keyword.objects.all().order_by("title"):
            prefix = "+" if str(keyword.id) not in self._ids else "-"
            links += ("<a href='#'>%s%s</a>" % (prefix, str(keyword)))
        rendered += mark_safe("<p class='keywords-field'>%s</p>" % links)
        return rendered