Support string colors

[aprilnovak]

I'd like to highlight some std::string cells in the VariadicTable with colors. For the lines I want to highlight, I'm doing

COLOR_YELLOW << "stuff" << COLOR_DEFAULT

and for all other lines, I'm doing

COLOR_DEFAULT << "stuff" << COLOR_DEFAULT

It seems like the logic for deciding the length of the horizontal bars is not consistent with the cell widths, because the table looks like the following.