If a table has explicit column width information *and* the
content extends beyond the `--columns` width, we need to
adjust the widths of the pipe separators to encode this width
information.
Closes#7847.
Instead rely on the markdown writer with appropriate extensions.
Export writeCommonMark variant from Markdown writer.
This changes a few small things in rendering markdown,
e.g. w/r/t requiring backslashes before spaces inside
super/subscripts.
...even when we must lose width information.
All in all this seems to be people's preferred behavior, even though it
is slightly lossier.
Closes#2608.
Closes#4497.
