Write title (optional).

def wr_title(self, worksheet, row_idx=0):
        """Write title (optional)."""
        if self.vars.title is not None:
            # Title is one line
            if isinstance(self.vars.title, str):
                return self.wr_row_mergeall(worksheet, self.vars.title, self.fmt_hdr, row_idx)
            # Title is multi-line
            else:
                ridx = row_idx
                for title_line in self.vars.title:
                    ridx = self.wr_row_mergeall(worksheet, title_line, self.fmt_hdr, ridx)
                return ridx
        return row_idx