Make --id-prefix affect footnote IDs in markdown writer.

Closes #614.

README

@@ -416,8 +416,9 @@ Options affecting specific writers
 
 `--id-prefix`=*STRING*
 :   Specify a prefix to be added to all automatically generated identifiers
-    in HTML and DocBook output.  This is useful for preventing duplicate
-    identifiers when generating fragments to be included in other pages.
+    in HTML and DocBook output, and to footnote numbers in markdown output.
+    This is useful for preventing duplicate identifiers when generating
+    fragments to be included in other pages.
 
 `-T` *STRING*, `--title-prefix=`*STRING*
 :   Specify *STRING* as a prefix at the beginning of the title

src/Text/Pandoc/Options.hs

@@ -215,6 +215,7 @@ data WriterOptions = WriterOptions
   , writerColumns          :: Int    -- ^ Characters in a line (for text wrapping)
   , writerEmailObfuscation :: ObfuscationMethod -- ^ How to obfuscate emails
   , writerIdentifierPrefix :: String -- ^ Prefix for section & note ids in HTML
+                                     -- and for footnote marks in markdown
   , writerSourceDirectory  :: FilePath -- ^ Directory path of 1st source file
   , writerUserDataDir      :: Maybe FilePath -- ^ Path of user data directory
   , writerCiteMethod       :: CiteMethod -- ^ How to print cites

src/Text/Pandoc/Writers/Markdown.hs

@@ -174,7 +174,7 @@ notesToMarkdown opts notes =
 noteToMarkdown :: WriterOptions -> Int -> [Block] -> State WriterState Doc
 noteToMarkdown opts num blocks = do
   contents  <- blockListToMarkdown opts blocks
-  let num' = text $ show num
+  let num' = text $ writerIdentifierPrefix opts ++ show num
   let marker = if isEnabled Ext_footnotes opts
                   then text "[^" <> num' <> text "]:"
                   else text "[" <> num' <> text "]"
@@ -655,7 +655,7 @@ inlineToMarkdown opts (Image alternate (source, tit)) = do
 inlineToMarkdown opts (Note contents) = do
   modify (\st -> st{ stNotes = contents : stNotes st })
   st <- get
-  let ref = show $ (length $ stNotes st)
+  let ref = text $ writerIdentifierPrefix opts ++ show (length $ stNotes st)
   if isEnabled Ext_footnotes opts
-     then return $ "[^" <> text ref <> "]"
-     else return $ "[" <> text ref <> "]"
+     then return $ "[^" <> ref <> "]"
+     else return $ "[" <> ref <> "]"