Allow formatted bibliography to be placed in metadata fields.

This modifies `processCitations` so that pandoc will look not just
in the document body but in metadata for a Div with id `refs` in
which to place the formatted bibliography.

Thus, one can include a metadata field, say `refs`, whose content
is an empty div with id `refs`, and the formatted bibliography
will be put into this metadata field.  It may then be interpolated
into a template using the variable `refs`.

Closes #7969.

Closes #526 by providing a way to interpolate references into
a template.

MANUAL.txt

@@ -5975,6 +5975,19 @@ The bibliography will be inserted after this heading.  Note that
 the `unnumbered` class will be added to this heading, so that the
 section will not be numbered.
 
+If you want to put the bibliography into a variable in your
+template, one way to do that is to put the div with id `refs`
+into a metadata field, e.g.
+
+    ---
+    refs: |
+       ::: {#refs}
+       :::
+    ...
+
+You can then put the variable `$refs$` into your template where
+you want the bibliography to be placed.
+
 ## Including uncited items in the bibliography
 
 If you want to include items in the bibliography without actually

src/Text/Pandoc/Citeproc.hs

@@ -108,8 +108,8 @@ processCitations (Pandoc meta bs) = do
          evalState (walkM insertResolvedCitations $ Pandoc meta' bs)
          $ cits
   return $ walk removeQuoteSpan
-         $ Pandoc meta''
-         $ insertRefs refkvs classes meta'' (B.toList bibs) bs'
+         $ insertRefs refkvs classes (B.toList bibs)
+         $ Pandoc meta'' bs'
 
 removeQuoteSpan :: Inline -> Inline
 removeQuoteSpan (Span ("",["csl-quoted"],[]) xs) = Span nullAttr xs
@@ -461,23 +461,24 @@ isYesValue _ = False
 -- if document contains a Div with id="refs", insert
 -- references as its contents.  Otherwise, insert references
 -- at the end of the document in a Div with id="refs"
-insertRefs :: [(Text,Text)] -> [Text] -> Meta -> [Block] -> [Block] -> [Block]
-insertRefs _ _ _  []   bs = bs
-insertRefs refkvs refclasses meta refs bs =
+insertRefs :: [(Text,Text)] -> [Text] -> [Block] -> Pandoc -> Pandoc
+insertRefs _ _ [] d = d
+insertRefs refkvs refclasses refs (Pandoc meta bs) =
   if isRefRemove meta
-     then bs
-     else case runState (walkM go bs) False of
-               (bs', True) -> bs'
-               (_, False)
-                 -> case refTitle meta of
+     then Pandoc meta bs
+     else case runState (walkM go (Pandoc meta bs)) False of
+               (d', True) -> d'
+               (Pandoc meta' bs', False)
+                 -> Pandoc meta' $
+                    case refTitle meta of
                       Nothing ->
-                        case reverse bs of
+                        case reverse bs' of
                           Header lev (id',classes,kvs) ys : xs ->
                             reverse xs ++
                             [Header lev (id',addUnNumbered classes,kvs) ys,
                              Div ("refs",refclasses,refkvs) refs]
-                          _ -> bs ++ [refDiv]
-                      Just ils -> bs ++
+                          _ -> bs' ++ [refDiv]
+                      Just ils -> bs' ++
                         [Header 1 ("bibliography", ["unnumbered"], []) ils,
                          refDiv]
   where