Return a string which represents a horizontal line.

def _build_line(colwidths, colaligns, linefmt):
    "Return a string which represents a horizontal line."
    if not linefmt:
        return None
    if hasattr(linefmt, "__call__"):
        return linefmt(colwidths, colaligns)
    else:
        begin, fill, sep,  end = linefmt
        cells = [fill*w for w in colwidths]
        return _build_simple_row(cells, (begin, sep, end))