RDiscount to support Jekyll 4.0 via plugin

[davidfstr]

Jekyll 4.0 removes support for the RDiscount Markdown processor from the core, relying on the wider community to create a plugin for processors like RDiscount. This issue tracks efforts to create a jekyll-rdiscount processor plugin for Jekyll 4.0 so that RDiscount can again be used in the latest version of Jekyll.

Requirements:

• Insofar as possible, maintain support for the same API that RDiscount used when integrating with Jekyll 3.x and below. In particular the following configuration blob should still work:

Design / Tasks:

• Create repository for plugin at davidfstr/jekyll-rdiscount.
• Depend on rdiscount >= 2.2.0.2 (the current version of RDiscount, to ease migration).
• Define a class Jekyll::Converters::Markdown::rdiscount class, with that exact case, to allow the following config in _config.yml to work:
  • markdown: rdiscount (equivalent to the old API)
• Plugin does read rdiscount: top-level section from _config.yml (equivalent to the old API) to specify things like flags.
• Write docs, for plugin itself:
  • Based on the new CommonMark plugin, need to install in Jekyll 4.0 by also adding the following to a Jekyll project's Gemfile:

group :jekyll_plugins do
  gem 'jekyll-rdiscount'
end

• Add continuous integration, probably with GitHub Actions, with matrix of Ruby versions and Jekyll versions.
  • Perhaps look at CommonMark plugin for inspiration on the matrix.
• Publish plugin.
• Announce plugin on blog/Twitter/etc.
• Draft/shepherd PR that mentions RDiscount plugin in the following documentation page from Jekyll:
  • https://jekyllrb.com/docs/configuration/markdown/#custom-markdown-processors
  • https://jekyllrb.com/docs/upgrading/3-to-4/