OpenDocument writer: add custom-style "Abstract" in metadata abstract.

This ensures that the abstract is rendered with style Abstract.

src/Text/Pandoc/Writers/OpenDocument.hs

@@ -28,6 +28,7 @@ import Text.Pandoc.BCP47 (Lang (..), parseBCP47)
 import Text.Pandoc.Class.PandocMonad (PandocMonad, report, translateTerm,
                                       setTranslations, toLang)
 import Text.Pandoc.Definition
+import qualified Text.Pandoc.Builder as B
 import Text.Pandoc.Logging
 import Text.Pandoc.Options
 import Text.DocLayout
@@ -223,12 +224,18 @@ writeOpenDocument opts (Pandoc meta blocks) = do
   let colwidth = if writerWrapText opts == WrapAuto
                     then Just $ writerColumns opts
                     else Nothing
+  let meta' = case lookupMetaBlocks "abstract" meta of
+                [] -> meta
+                xs -> B.setMeta "abstract"
+                        (B.divWith ("",[],[("custom-style","Abstract")])
+                          (B.fromList xs))
+                        meta
   ((body, metadata),s) <- flip runStateT
         defaultWriterState $ do
            m <- metaToContext opts
                   (blocksToOpenDocument opts)
                   (fmap chomp . inlinesToOpenDocument opts)
-                  meta
+                  meta'
            b <- blocksToOpenDocument opts blocks
            return (b, m)
   let styles   = stTableStyles s ++ stParaStyles s ++ formulaStyles ++