If the table lacks a header, the header row should be an empty
list. Previously we got a list of empty cells, which caused
an empty header to be emitted instead of no header. In LaTeX/PDF
output that meant we got a double top line with space between.
@tarleb @despres - please let me know if this is problematic
for some reason I'm not grasping.
- Writers.Native is now adapted to the new Table type.
- Inline captions should now be conditionally wrapped in a Plain, not
a Para block.
- The toLegacyTable function now lives in Writers.Shared.
