ANN: pandoc 2.16

[John MacFarlane]

I'm pleased to announce the release of pandoc 2.16,
available in the usual places:

Binary packages & changelog:
https://github.com/jgm/pandoc/releases/tag/2.16

Source & API documentation:
http://hackage.haskell.org/package/pandoc-2.16

The reason for a release so soon on the heels of 2.15 is
to fix some regressions in Lua filter support that came
with the transition to hslua 2. Anyone using Lua filters
is advised to upgrade immediately.

The other major change is a switch from the HsYAML library
back to the yaml library for parsing YAML metadata. This
change makes YAML parsing 20X faster, so that it
should once again be possible to use large bibliographies in
YAML format. For the most part this change should be
invisible to users, but note the following:

- Unlike HsYAML, yaml parses 'Y', 'N', 'Yes', 'No', 'On',
and 'Off' as boolean values. Users may need to quote
these when they are meant to be interpreted as strings.
Similarly, 'null' is parsed as a YAML null value (and
will be treated as an empty string by pandoc rather than
the string 'null'). Quoting it will force it to be
interpreted as a string.

- Pandoc should now fail with an informative error when
you don't escape your YAML properly, instead of failing
silently and parsing the section as regular text.

The API changes: yaml has no separate FromYAML typeclass,
just using FromJSON instead, so all the FromYAML instances
for pandoc types have been dropped. In addition, the
JSON encodings of a few types have been changed (LineEnding,
HTMLSlideVariant, ReaderOptions, Extension, Extensions,
Opt). (These changes shouldn't affect JSON filters, because
they don't concern the types defined in pandoc-types.)

Thanks to all who contributed, especially Albert Krewinkel
for his work on the Lua subsystem and Tristan Stenner for
some improvements to the docx writer.