RST reader: recurse into bodies of unknown directives.

In most cases it's better to preserve the content than
to emit it.  This isn't guaranteed to have good results;
it will fail spectacularly for unknown raw or verbatim
directives.

See #3432.

src/Text/Pandoc/Readers/RST.hs

@@ -755,7 +755,8 @@ directive' = do
         other     -> do
             pos <- getPosition
             logMessage $ SkippedContent (".. " ++ other) pos
-            return mempty
+            bod <- parseFromString parseBlocks $ top ++ "\n\n" ++ body'
+            return $ B.divWith ("",[other],[]) bod
 
 tableDirective :: PandocMonad m
                => String -> [(String, String)] -> String -> RSTParser m Blocks