Set Ok (success) finalizer to a spinner.

def ok(self, text=u"OK", err=False):
        """Set Ok (success) finalizer to a spinner."""
        # Do not display spin text for ok state
        self._text = None

        _text = to_text(text) if text else u"OK"
        err = err or not self.write_to_stdout
        self._freeze(_text, err=err)