ANN: pandoc 2.13

[John MacFarlane]

I'm pleased to announce the release of pandoc 2.13,
available in the usual places:

Binary packages & changelog:
https://github.com/jgm/pandoc/releases/tag/2.13

Source & API documentation:
http://hackage.haskell.org/package/pandoc-2.13

This is a major version bump because we took the opportunity
to clean up some cruft in the API. But the API changes are minor
and should not affect must users of the library.

API changes:

* Text.Pandoc.Shared: remove backslashEscapes and escapeStringUsing.
* Text.Pandoc.Shared: remove ToString and ToText typeclasses, which were
used when we were making the transition from Stirng to Text.
* Text.Pandoc.Parsing: change the type of readWithM, so that it
always expects Text input instead of being polymorphic.
* Text.Pandoc.Parsing: remove the F type synonym.
* Text.Pandoc.Readers.Metadata: eport yamlMetaBlock.
* Text.Pandoc.Readers.Metadata: make yamlBsToMeta and yamlBsToRefs
polymorphic on teh parser state.
* Text.Pandoc.Logging: add DuplicateAttribute contsructor for LogMessage.

Some highlights of the user-facing changes:

- yaml_metadata_block extension now works for commonmark (though in a more
limited way than with markdown).
- template partials are sought in the default templates if they
aren't supplied with a custom template.
- nocite works better now with --biblatex and --natbib options.
- the tex_math_backslash extension works again.
- Haskell syntax highlighting works again.

Thanks to everyone who contributed, especially Albert Krewinkel and
new contributors Bodigrim, Erik Rask, and Florian Kohrt.