Add commonmark-code-blocks filter for producing CommonMark-suggested HTML for fenced code blocks

commonmark-code-blocks/Makefile

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+DIFF := diff --ignore-all-space --ignore-blank-lines -u
+PANDOC ?= pandoc
+
+.PHONY: test
+test:
+	@$(PANDOC) --lua-filter=commonmark-code-blocks.lua -f commonmark -t html sample.md | \
+		$(DIFF) expected.html -
+
+.PHONY: all
+all: test
+

commonmark-code-blocks/README.md

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+# commonmark-code-blocks 
+
+This filter produces code blocks with language CSS class format
+suggested by the CommonMark specification ("language-$lang").
+
+The CommonMark spec suggests that converting ````somelang` to HTML
+should produce `<code class="language-somelang">`,
+as described in [example 141](https://spec.commonmark.org/0.30/#example-141).
+
+That's what most CommonMark implementations do.
+Pandoc, however, outputs `<code class="somelang">` instead.
+
+That can be a problem for syntax highlighters that expect
+`language-*` classes and use them to determine the language.
+
+This filter takes over the code block rendering process
+to produce CommonMark-style output.
+
+## Usage
+
+This filter does not require any setup or external tools.
+Just run pandoc with `--lua-filter=commonmark-code-blocks.lua`.
+

commonmark-code-blocks/commonmark-code-blocks.lua

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+--[[
+
+commonmark-code-blocks -- produces code blocks with language CSS class format
+suggested by the CommonMark specification ("language-$lang").
+
+MIT License
+
+Copyright (c) 2021 Daniil Baturin
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+]]
+
+--[[
+
+The CommonMark spec suggests that 
+"```somelang" should produce <code class="language-somelang">
+
+That's what most CommonMark implementations do.
+
+Pandoc, however, outputs <code class="somelang">
+
+That can be a problem for syntax highlighters that expect
+"language-*" classes and use them to determine the language.
+
+This filter takes over the code block rendering process
+to produce CommonMark-style output.
+
+]]
+
+function CodeBlock(block)
+  if FORMAT:match 'html' then
+    local lang_attr = ""
+    if (#block.classes > 0) then
+      lang_attr = string.format("class=\"language-%s\"", block.classes[1])
+    else
+      -- Ignore code blocks where language is not specified
+    end
+
+    local code = block.text:gsub("&", "&amp;"):gsub("<", "&lt;"):gsub(">", "&gt;")
+
+    local html = string.format('<pre><code %s>%s</code></pre>', lang_attr, code)
+    return pandoc.RawBlock('html', html)
+  end
+end
+

commonmark-code-blocks/expected.html

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+<pre><code class="language-basic">10 PRINT "HELLO WORLD"</code></pre>
+

commonmark-code-blocks/sample.md

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+```basic
+10 PRINT "HELLO WORLD"
+```