Allow citation-abbreviations in defaults file.

MANUAL.txt

@@ -1533,6 +1533,7 @@ csl: ieee
 bibliography:
 - foobar.bib
 - barbaz.json
+citation-abbreviations: abbrevs.json
 
 # Filters will be assumed to be Lua filters if they have
 # the .lua extension, and json filters otherwise.  But

src/Text/Pandoc/App/Opt.hs

@@ -367,6 +367,11 @@ doOpt (k',v) = do
                                                     foldr addItem o xs)
           <|>
           (parseYAML v >>= \(x :: Text) -> return $ \o -> addItem x o)
+    "citation-abbreviations" ->
+      parseYAML v >>= \x ->
+             return (\o -> o{ optMetadata =
+                                addMeta "citation-abbreviations" (T.unpack x)
+                                  (optMetadata o) })
     "ipynb-output" ->
       parseYAML v >>= \x -> return (\o -> o{ optIpynbOutput = x })
     "include-before-body" ->