In the past, the docx reader wrote an empty header as an empty list. It
should have the same width as a row (and be filled with empty cells).
(Note that I've reordered the code here slightly to get rid of a call to
`head`. It wasn't unsafe because it tested for null, but it was a bit of
a smell.)
Renamed some tests, introducing subsidiary directories
for fb2, docx, epub.
Cleaned up tests in cabal file.
Combined dokuwiki-writer and dokuwiki_inline_formatting tests.
