Add raw_markdown extension affecting ipynb reader.

Specifying `-f ipynb+raw_markdown` will cause Markdown cells
to be represented as raw Markdown blocks, instead of being
parsed.  This is not what you want when going from `ipynb`
to other formats, but it may be useful when going from `ipynb`
to Markdown or to `ipynb`, to avoid semantically insignificant
changes in the contents of the Markdown cells that might
otherwise be introduced.

Closes #5408.

MANUAL.txt

@@ -2910,6 +2910,14 @@ input formats
 In the `muse` input format, this enables Text::Amuse
 extensions to Emacs Muse markup.
 
+#### Extension: `raw_markdown` ####
+
+In the `ipynb` input format, this causes Markdown cells
+to be included as raw Markdown blocks (allowing lossless
+round-tripping) rather than being parsed.  Use this only
+when you are targetting `ipynb` or a markdown-based
+output format.
+
 #### Extension: `citations` {#org-citations}
 
 Some aspects of [Pandoc's Markdown citation syntax](#citations) are also accepted

src/Text/Pandoc/Extensions.hs

@@ -133,6 +133,7 @@ data Extension =
     | Ext_raw_attribute           -- ^ Allow explicit raw blocks/inlines
     | Ext_raw_html            -- ^ Allow raw HTML
     | Ext_raw_tex             -- ^ Allow raw TeX (other than math)
+    | Ext_raw_markdown        -- ^ Parse markdown in ipynb as raw markdown
     | Ext_shortcut_reference_links -- ^ Shortcut reference links
     | Ext_simple_tables       -- ^ Pandoc-style simple tables
     | Ext_smart               -- ^ "Smart" quotes, apostrophes, ellipses, dashes
@@ -442,7 +443,8 @@ getAllExtensions f = universalExtensions <> getAll f
   getAll "markdown_mmd"      = allMarkdownExtensions
   getAll "markdown_github"   = allMarkdownExtensions
   getAll "markdown"          = allMarkdownExtensions
-  getAll "ipynb"             = allMarkdownExtensions
+  getAll "ipynb"             = allMarkdownExtensions <> extensionsFromList
+    [ Ext_raw_markdown ]
   getAll "docx"            = extensionsFromList
     [ Ext_empty_paragraphs
     , Ext_styles

src/Text/Pandoc/Readers/Ipynb.hs

@@ -78,7 +78,11 @@ cellToBlocks opts lang c = do
   mapM_ addAttachment attachments
   case cellType c of
     Ipynb.Markdown -> do
-      Pandoc _ bs <- walk fixImage <$> readMarkdown opts source
+      bs <- if isEnabled Ext_raw_markdown opts
+               then return [RawBlock (Format "markdown") source]
+               else do
+                 Pandoc _ bs <- walk fixImage <$> readMarkdown opts source
+                 return bs
       return $ B.divWith ("",["cell","markdown"],kvs)
              $ B.fromList bs
     Ipynb.Heading lev -> do

src/Text/Pandoc/Writers/Markdown.hs

@@ -483,10 +483,10 @@ blockToMarkdown' opts b@(RawBlock f str) = do
   let renderEmpty = mempty <$ report (BlockNotRendered b)
   case variant of
     PlainText -> renderEmpty
-    _ | isEnabled Ext_raw_attribute opts -> rawAttribBlock
-      | f `elem` ["markdown", "markdown_github", "markdown_phpextra",
+    _ | f `elem` ["markdown", "markdown_github", "markdown_phpextra",
                   "markdown_mmd", "markdown_strict"] ->
             return $ literal str <> literal "\n"
+      | isEnabled Ext_raw_attribute opts -> rawAttribBlock
       | f `elem` ["html", "html5", "html4"] ->
             case () of
               _ | isEnabled Ext_markdown_attribute opts -> return $