Format table separator line.

def fmt_border(self, dimensions, t = 'm', border_style = 'utf8.a', border_formating = {}):
        """
        Format table separator line.
        """
        cells = []
        for column in dimensions:
            cells.append(self.bchar('h', t, border_style) * (dimensions[column] + 2))

        border = '{}{}{}'.format(self.bchar('l', t, border_style), self.bchar('m', t, border_style).join(cells), self.bchar('r', t, border_style))
        return self.fmt_text(border, **border_formating)