v0.4.0

This is a major release with probably significant backwards-incompatible changes (mostly in the "Changes to output format" section).

Features

• Org mode 9.7 support (#9, thanks to tcahill for the contribution):
  • Handle deferred properties and the :standard-properties vector via org-element-properties-map
  • Skip private properties (names containing --)
  • Stop exporting :post-affiliated, :robust-begin, and :robust-end
  • Export tests now compare full document output against checked-in reference JSON. The single
    tests/test.org/test.json pair was replaced by a set of topic-focused fixtures in
    tests/export/ (one .org/.json pair per feature area), each with its own ERT test.
• New export options:
  • :json-postprocess (pretty, minimal, or nil): to control final JSON formatting (default
    :pretty).
  • :json-deterministic-refs (t or nil): Make element ref values deterministic
    (based on structural path in the parse tree) (default nil).

Changes to output format

• Omit the contents key from exported nodes when it is empty.
• Custom type-specific node properties/fields have been moved from inside "properties" to the
  outer level, to distinguish them from Org's builtin properties:
  • headline nodes: tags-all
  • link nodes: target-ref, is-internal, is-inline-image
  • timestamp nodes: start, end
• Node properties objects now export keys in alphabetical order.
• Omit default node property values from export. Set the :json-omit-default-property-values to
  nil to restore old behavior.
• Export single-character properties (headline priority) as strings instead of character codes.

Bug fixes

• Resolve Org 9.7 headline deferred properties (:archivedp, :footnote-section-p,
  etc.) with force-undefer when enumerating node properties. Shared deferred
  placeholders with auto-undefer-p nil were previously exported as true.
• Fix improper quoting in cl-case link-type branches.
• Fix incorrect argument order in ox-json--type-error calls.
• Fix wrong-number-of-arguments crash during JSON post-processing on Emacs
  versions before 28.1, where json-pretty-print-buffer takes no arguments.

Other

• Raise minimum supported versions of Emacs from 26.1 to 27.1 and Org from 9 to 9.4.
• Split into multiple source files.
• Adopt Eask for dependency management, byte compilation, and CI.
• Add DEVELOPER.md developer guide.
• Add test coverage reporting via undercover.
• Makefile refactored as a thin facade over Eask.
• Use strict bool parsing by default (properties marked as bools must be nil or t).
• Add scripts/json-to-markdown.py and scripts/json-to-html.py to render exported JSON as
  human-readable Markdown/HTML for reviewing fixtures (make update-exports-pretty).
• Add scripts/eask-docker.sh and EASK_DOCKER Makefile support for testing against specific
  Emacs versions in a silex/emacs container.