Allow --list-extensions to take an optional FORMAT argument.

This lists the extensions set by default for the selected
FORMAT.

MANUAL.txt

@@ -387,11 +387,11 @@ General options
 
 :   List supported output formats, one per line.
 
-`--list-extensions`
+`--list-extensions`[`=`*FORMAT*]
 
 :   List supported Markdown extensions, one per line, followed
     by a `+` or `-` indicating whether it is enabled by default
-    in pandoc's Markdown.
+    in *FORMAT* (defaulting to pandoc's Markdown).
 
 `--list-highlight-languages`
 
@@ -1096,7 +1096,7 @@ of the following options.
     The *URL* should point to the `MathJax.js` load script.
     If a *URL* is not provided, a link to the Cloudflare CDN will
     be inserted.
- 
+
 `--mathml`
 
 :   Convert TeX math to [MathML] (in `epub3`, `docbook4`, `docbook5`, `jats`,

src/Text/Pandoc/App.hs

@@ -1589,15 +1589,17 @@ options =
                  ""
 
     , Option "" ["list-extensions"]
-                 (NoArg
-                  (\_ -> do
+                 (OptArg
+                  (\arg _ -> do
+                     let exts = getDefaultExtensions (fromMaybe "markdown" arg)
                      let showExt x = drop 4 (show x) ++
-                                       if extensionEnabled x pandocExtensions
+                                       if extensionEnabled x exts
                                           then " +"
                                           else " -"
                      mapM_ (UTF8.hPutStrLn stdout . showExt)
                                ([minBound..maxBound] :: [Extension])
-                     exitSuccess ))
+                     exitSuccess )
+                  "FORMAT")
                  ""
 
     , Option "" ["list-highlight-languages"]