markdown writer: formatted spans mess up table formatting

|short |short's description              |
|------|---------------------------------|
|`longer`|a longer description or something|
|*emph*|emphatically                     |
|**bold**|emboldened                       |

that was meant to be a headerless table; but whatever: this also illustrates the problem.  The column is only wide enough to accommodate the strings without their backticks, stars etc.  It's valid markdown, parses fine, just doesn't look good in plain text form.