jgm/pandoc: pandoc 2.9

Text.Pandoc.Templates [API change]

• Add Monad wrappers WithDefaultPartials and WithPartials. Wrapping these around an instance of PandocMonad gives us different instances of TemplateMonad, with different search behavior in retrieving partials. To compile a template and limit partial search to pandoc's data files, use runWithDefaultPartials (compileTemplate ...). To compile a template and allow partials to be found locally (either on the file system or via HTTP, in the event that the main template has an absolute URL), ue runWithPartials (compileTemplate ...).
• Export getTemplate, which seeks a template locally, or via HTTP if the template has an absolute URL, falling back to the data files if not found.
• Export compileDefaultTemplate – does getDefaultTemplate and compiles the result, raising an error on failure.

Text.Pandoc.Class [API change]

• Remove TemplateMonad instances for PandocIO and PandocPure. These were too limiting and caused a bug whereby a local partial could be used even when the default template was requested. We now rely on instances provided in the Templates module.

Text.Pandoc.App.OutputSettings: Simplify template retrieval code.

ConTeXt template: Adjust to title formatting (#5949, Denis Maier). Add \setupinterlinespace to title, subtitle, date and author elements: otherwise longer titles that run over multiple lines will look squashed as \tfd etc. won't adapt the line spacing to the font size.

reveal.js template: Add title-slide-attributes variable (#5981, Frederik Elwert).

More informative JSON parse error (#5973).

Use external emojis package (forked from pandoc). Removed emoji data in Text.Pandoc.Emoji.

Fix regression in makeSections (#5965). Previously hierarchicalize (the ancestor of makeSections) would put header attributes on the containing Div. In 2.8 this behavior changed, which broke some tools depending on pandoc. Here we roll back this change, so that attributes again migrate from the header to the containing Div when makeSections is run. Note that attributes are retained on the header as well (unlike before) – with the exception of the id attribute, which of course cannot be duplicated.

Fix --toc-depth regression in 2.8 (#5967).

Use doctemplates 0.8. Rename template 'filters' as 'pipes' to avoid confusion with the other notion of filter used by pandoc.

Fix README.md so that relative links from manual become absolute. Previously they'd be broken links when viewed on GitHub or Hackage. So we add the base URL for the pandoc manual.

Document display math syntax in manual.