* Markdown writer now includes a blank line at the end
of the row in a single-row multiline table, to prevent it from being
interpreted as a simple table. Closes#4578.
* Markdown reader does a better job computing the relative width of
the last column in a multiline table, so we can round-trip tables
without constantly shrinking the last column.
Currently the support for the `.. table` directive is a bit
limited; we don't yet support the `widths` field. But at least
you can have a proper captioned table.
