Lua: support extensions in custom writers

Custom writers can define the extensions that they support via the
global `writer_extensions`. The variable's value must be a table with
all supported extensions as keys, and their default status as values.

E.g., the below specifies that the writer support the extensions `smart`
and `sourcepos`, but only the `smart` extension is enabled by default:

    writer_extensions = {
      smart = true,
      sourcepos = false,
    }

2 files changed, 6 insertions, 3 deletions

diff --git a/src/Text/Pandoc/App/OutputSettings.hs b/src/Text/Pandoc/App/OutputSettings.hs
index 20321f52a..ab224cd7f 100644
--- a/src/Text/Pandoc/App/OutputSettings.hs
+++ b/src/Text/Pandoc/App/OutputSettings.hs
@@ -107,7 +107,9 @@ optToOutputSettings scriptingEngine opts = do
   (writer, writerExts) <-
     if "lua" `T.isSuffixOf` format
     then do
-      (, mempty) <$> engineWriteCustom scriptingEngine (T.unpack format)
+      (w, extsConf) <- engineWriteCustom scriptingEngine (T.unpack format)
+      wexts         <- Format.applyExtensionsDiff extsConf flvrd
+      return (w, wexts)
     else do
       if optSandbox opts
       then case runPure (getWriter flvrd) of
diff --git a/src/Text/Pandoc/Scripting.hs b/src/Text/Pandoc/Scripting.hs
index 09b19f12e..0defa2f77 100644
--- a/src/Text/Pandoc/Scripting.hs
+++ b/src/Text/Pandoc/Scripting.hs
@@ -18,10 +18,11 @@ where
 import Control.Monad.Except (throwError)
 import Control.Monad.IO.Class (MonadIO)
 import Data.Text (Text)
-import Text.Pandoc.Definition (Pandoc)
 import Text.Pandoc.Class.PandocMonad (PandocMonad)
+import Text.Pandoc.Definition (Pandoc)
 import Text.Pandoc.Error (PandocError (PandocNoScriptingEngine))
 import Text.Pandoc.Filter.Environment (Environment)
+import Text.Pandoc.Format (ExtensionsConfig)
 import Text.Pandoc.Readers (Reader)
 import Text.Pandoc.Writers (Writer)
 
@@ -39,7 +40,7 @@ data ScriptingEngine = ScriptingEngine
     -- ^ Function to parse input into a 'Pandoc' document.
 
   , engineWriteCustom :: forall m. (PandocMonad m, MonadIO m)
-                      => FilePath -> m (Writer m)
+                      => FilePath -> m (Writer m, ExtensionsConfig)
     -- ^ Invoke the given script file to convert to any custom format.
   }