Org writer: prefix footnote numbers with fn:

Unprefixed numbers where used by older org-mode versions, but are no
longer supported.

src/Text/Pandoc/Writers/Org.hs

@@ -92,7 +92,7 @@ notesToOrg notes =
 noteToOrg :: Int -> [Block] -> State WriterState Doc
 noteToOrg num note = do
   contents <- blockListToOrg note
-  let marker = "[" ++ show num ++ "] "
+  let marker = "[fn:" ++ show num ++ "] "
   return $ hang (length marker) (text marker) contents
 
 -- | Escape special characters for Org.
@@ -373,7 +373,7 @@ inlineToOrg (Note contents) = do
   notes <- get >>= (return . stNotes)
   modify $ \st -> st { stNotes = contents:notes }
   let ref = show $ (length notes) + 1
-  return $ "[" <> text ref <> "]"
+  return $ "[fn:" <> text ref <> "]"
 
 orgPath :: String -> String
 orgPath src =

tests/writer.org

@@ -819,37 +819,37 @@ Here is a movie [[file:movie.jpg]] icon.
   :CUSTOM_ID: footnotes
   :END:
 
-Here is a footnote reference,[1] and another.[2] This should /not/ be a
+Here is a footnote reference,[fn:1] and another.[fn:2] This should /not/ be a
 footnote reference, because it contains a space.[\^my note] Here is an inline
-note.[3]
+note.[fn:3]
 
 #+BEGIN_QUOTE
-  Notes can go in quotes.[4]
+  Notes can go in quotes.[fn:4]
 #+END_QUOTE
 
-1. And in list items.[5]
+1. And in list items.[fn:5]
 
 This paragraph should not be part of the note, as it is not indented.
 
-[1] Here is the footnote. It can go anywhere after the footnote reference. It
-    need not be placed at the end of the document.
+[fn:1] Here is the footnote. It can go anywhere after the footnote reference.
+       It need not be placed at the end of the document.
 
-[2] Here's the long note. This one contains multiple blocks.
+[fn:2] Here's the long note. This one contains multiple blocks.
 
-    Subsequent blocks are indented to show that they belong to the footnote
-    (as with list items).
+       Subsequent blocks are indented to show that they belong to the footnote
+       (as with list items).
 
-    #+BEGIN_EXAMPLE
-          { <code> }
-    #+END_EXAMPLE
+       #+BEGIN_EXAMPLE
+             { <code> }
+       #+END_EXAMPLE
 
-    If you want, you can indent every line, but you can also be lazy and just
-    indent the first line of each block.
+       If you want, you can indent every line, but you can also be lazy and
+       just indent the first line of each block.
 
-[3] This is /easier/ to type. Inline notes may contain
-    [[http://google.com][links]] and =]= verbatim characters, as well as
-    [bracketed text].
+[fn:3] This is /easier/ to type. Inline notes may contain
+       [[http://google.com][links]] and =]= verbatim characters, as well as
+       [bracketed text].
 
-[4] In quote.
+[fn:4] In quote.
 
-[5] In list.
+[fn:5] In list.