Format box separator line.

def fmt_border(self, width, t = 'm', border_style = 'utf8.a', border_formating = {}):
        """
        Format box separator line.
        """
        border = self.bchar('l', t, border_style) + (self.bchar('h', t, border_style) * (width-2)) + self.bchar('r', t, border_style)
        return self.fmt_text(border, **border_formating)