Markdown writer - support new table features

[jgm]

Add support for (at least some of) the new table features introduced in pandoc-types/pull/66.

This must wait for syntax decisions: see issue #6317

[bwl21]

Even if it has to wait for #6317 I would like to add the request that Markdown writer has an option to write the table in the same style as in the input file.

Usecase: we should be able to use different table formats in one document (as each of the formats has its pros and cons). If we use pandoc as a markdown beautifier - the table formats should not be changed.

[jgm]

Update: the markdown reader now supports row/colspans. It would be good if the writer could at least support that (see #9289), even if we don't have syntax decisions for things like row headers.