Define a `meta-json` variable for all writers.

This contains a JSON version of all the metadata, in the
format selected for the writer.

So, for example, to get just the YAML metadata, you can
run pandoc with the following custom template:

    $meta-json$

Closes #2019.  The intent is to make it easier for static
site generators and other tools to get at the metadata.

2 files changed, 6 insertions, 3 deletions

diff --git a/src/Text/Pandoc/Writers/Markdown.hs b/src/Text/Pandoc/Writers/Markdown.hs
index 731fa86c4..9f06aa054 100644
--- a/src/Text/Pandoc/Writers/Markdown.hs
+++ b/src/Text/Pandoc/Writers/Markdown.hs
@@ -126,7 +126,8 @@ jsonToYaml (Object hashmap) =
                  | otherwise      -> (k' <> ":") $$ x
                (k', Object _, x)  -> (k' <> ":") $$ nest 2 x
                (_, String "", _)  -> empty
-               (k', _, x)         -> k' <> ":" <> space <> hang 2 "" x)
+               (k', _, x)  | k == "meta-json"  -> empty
+                           | otherwise -> k' <> ":" <> space <> hang 2 "" x)
        $ sortBy (comparing fst) $ H.toList hashmap
 jsonToYaml (Array vec) =
   vcat $ map (\v -> hang 2 "- " (jsonToYaml v)) $ V.toList vec
diff --git a/src/Text/Pandoc/Writers/Shared.hs b/src/Text/Pandoc/Writers/Shared.hs
index d94dbac46..865d10123 100644
--- a/src/Text/Pandoc/Writers/Shared.hs
+++ b/src/Text/Pandoc/Writers/Shared.hs
@@ -45,7 +45,8 @@ import Text.Pandoc.Options (WriterOptions(..))
 import qualified Data.HashMap.Strict as H
 import qualified Data.Map as M
 import qualified Data.Text as T
-import Data.Aeson (FromJSON(..), fromJSON, ToJSON (..), Value(Object), Result(..))
+import Data.Aeson (FromJSON(..), fromJSON, ToJSON (..), Value(Object), Result(..), encode)
+import Text.Pandoc.UTF8 (toStringLazy)
 import qualified Data.Traversable as Traversable
 import Data.List ( groupBy )
 
@@ -67,7 +68,8 @@ metaToJSON opts blockWriter inlineWriter (Meta metamap)
     renderedMap <- Traversable.mapM
                    (metaValueToJSON blockWriter inlineWriter)
                    metamap
-    return $ M.foldWithKey defField baseContext renderedMap
+    let metadata = M.foldWithKey defField baseContext renderedMap
+    return $ defField "meta-json" (toStringLazy $ encode metadata) metadata
   | otherwise = return (Object H.empty)
 
 metaValueToJSON :: Monad m