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.

README

@@ -1035,7 +1035,7 @@ depending on the output format, but include metadata fields as well as the follo
         ...
 
 `subtitle`
-:   document subtitle; also used as subject in PDF metadata
+:   document subtitle
 
 `abstract`
 :   document summary, included in LaTeX, ConTeXt, AsciiDoc, and Word docx
@@ -1064,6 +1064,9 @@ depending on the output format, but include metadata fields as well as the follo
 `body`
 :   body of document
 
+`meta-json`
+:   JSON representation of all of the document's metadata
+
 Language variables
 ------------------
 

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

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