Org reader: allow multiple #+SUBTITLE export settings

The values of all lines are read as inlines and collected in the
`subtitle` metadata field.

doc/org.md

@@ -51,7 +51,7 @@ occur.
 
   Pandoc follows the LaTeX exporter in that it allows markup in
   the description. In contrast, the Org-mode HTML exporter treats
-  reads the description as plain text.
+  the description as plain text.
 
 - LATEX_HEADER: arbitrary lines to add to the document's preamble.
   Contrary to Org-mode, these lines are not inserted before the
@@ -74,6 +74,9 @@ occur.
 
 - SUBTITLE: the document's subtitle; fully supported.
 
+  The content of this option is stored as inlines in the
+  `subtitle` metadata field.
+
 - HTML_HEAD: arbitrary lines to add to the HTML document's head;
   fully supported.
 

src/Text/Pandoc/Readers/Org/Meta.hs

@@ -79,6 +79,7 @@ exportSettingHandlers = Map.fromList
   , ("date"       , lineOfInlines `parseThen` setField "date")
   , ("description", lineOfInlines `parseThen` collectLines "description")
   , ("keywords"   , lineOfInlines `parseThen` collectLines "keywords")
+  , ("subtitle"   , lineOfInlines `parseThen` collectLines "subtitle")
   , ("title"      , lineOfInlines `parseThen` collectLines "title")
     -- LaTeX
   , ("latex_class", fmap pure anyLine `parseThen` setField "documentclass")

test/Tests/Readers/Org/Meta.hs

@@ -93,6 +93,13 @@ tests =
         in Pandoc meta mempty
       ]
 
+    , "Subtitle" =:
+      T.unlines [ "#+SUBTITLE: Your Life in"
+                , "#+SUBTITLE: /Plain/ Text"
+                ] =?>
+      let subtitle = "Your Life in" <> softbreak <> emph "Plain" <> " Text"
+      in Pandoc (setMeta "subtitle" (toMetaValue subtitle) nullMeta) mempty
+
     , "Keywords" =:
       T.unlines [ "#+KEYWORDS: pandoc, testing,"
                 , "#+KEYWORDS: Org"