RFR: JDK-8224257: fix issues in files generated by pandoc

[Jonathan Gibbons]

Please review an update to the tool used to post-process files generated 
by pandoc.

In HTML 5, it is not enough to put `scope="row` on selected cells in the 
table,
the cells must also be converted to `<th>`. To counteract any potential 
change
in the visual appearance, the style attribute for the cell is also updated.

Tested by doing a visual comparison with meld of the files generated 
before/after
the change.

-- Jon

JBS: https://bugs.openjdk.java.net/browse/JDK-8224257
Webrev: http://cr.openjdk.java.net/~jjg/8225489-pandoc-tables/webrev.00/
Docs: http://cr.openjdk.java.net/~jjg/8225489-pandoc-tables/docs/