[pandoc] Fix compatibility with Pandoc 1.10.
[pandoc] Make Pandoc JSON reader compatible with both 1.9 and 1.10 versions.
Add command line option for Pandoc JSON version.

Default to 1.10.
[pandoc] Support Pandoc JSON 1.9 format.
[pandoc] Execute saved copy of Pandoc 1.9 in corresponding tests.
[pandoc] Add a test for reading Pandoc 1.9 JSON format.
Merge branch 'pandoc-1.10'