Docx Reader: Read single para in table cell as plain

This makes to docx reader's native output fit with the way the markdown
reader understands its markdown output. Ie, as far as table cells go:

docx -> native == docx -> native -> markdown -> native

(This identity isn't true for other things outside of table cells, of
course).
Docx reader: update tests for new table behavior.
Merge pull request #1577 from jkr/plainCells

Plain cells