Markdown writer: default to using ATX headings.

Previously we used Setext (underlined) headings by default.
The default is now ATX (`##` style).

* Add the `--markdown-headings=atx|setext` option.
* Deprecate `--atx-headers`.
* Add constructor 'ATXHeadingInLHS` constructor to `LogMessage` [API change].
* Support `markdown-headings` in defaults files.
* Document new options in MANUAL.

Closes #6662.

1 files changed, 18 insertions, 1 deletions

diff --git a/src/Text/Pandoc/App/CommandLineOptions.hs b/src/Text/Pandoc/App/CommandLineOptions.hs
index a82d8380e..661c6e06d 100644
--- a/src/Text/Pandoc/App/CommandLineOptions.hs
+++ b/src/Text/Pandoc/App/CommandLineOptions.hs
@@ -529,9 +529,26 @@ options =
 
     , Option "" ["atx-headers"]
                  (NoArg
-                  (\opt -> return opt { optSetextHeaders = False } ))
+                  (\opt -> do
+                    deprecatedOption "--atx-headers"
+                      "use --markdown-headings=atx"
+                    return opt { optSetextHeaders = False } ))
                  "" -- "Use atx-style headers for markdown"
 
+    , Option "" ["markdown-headings"]
+                  (ReqArg
+                    (\arg opt -> do
+                      headingFormat <- case arg of
+                        "setext" -> pure True
+                        "atx" -> pure False
+                        _ -> E.throwIO $ PandocOptionError $ T.pack
+                          ("Unknown markdown heading format: " ++ arg ++
+                            ". Expecting atx or setext")
+                      pure opt { optSetextHeaders = headingFormat }
+                    )
+                  "setext|atx")
+                  ""
+
     , Option "" ["listings"]
                  (NoArg
                   (\opt -> return opt { optListings = True }))