Implemented Ext_header_attributes.

This allows explicit attributes to be put on headers, using
a syntax like that for code blocks:

  {#id .class .class k=v k=v}

README

@@ -872,10 +872,31 @@ wrapping). Consider, for example:
 
 ### Header identifiers in HTML, LaTeX, and ConTeXt ###
 
-**Extension: `header_identifiers`**
+**Extension: `header_attributes`**
 
-Each header element in pandoc's HTML and ConTeXt output is given a
-unique identifier. This identifier is based on the text of the header.
+Headers can be assigned attributes using this syntax at the end
+of the line containing the header text:
+
+    {#identifier .class .class key=value key=value}
+
+Although this syntax allows assignment of classes and key/value attributes,
+only identifiers currently have any affect in the writers (and only in some
+writers: HTML, LaTeX, ConTeXt, Textile, AsciiDoc).  Thus, for example,
+the following headers will all be assigned the identifier `foo`:
+
+    # My header {#foo}
+
+    ## My header ##    {#foo}
+
+    My other header   {#foo}
+    ---------------
+
+(This syntax is compatible with [PHP Markdown Extra].)
+
+**Extension: `auto_identifiers`**
+
+A header without an explicitly specified identifier will be
+automatically assigned a unique identifier based on the header text.
 To derive the identifier from the header text,
 
   - Remove all formatting, links, etc.

src/Text/Pandoc/Options.hs

@@ -91,6 +91,7 @@ data Extension =
     | Ext_literate_haskell    -- ^ Enable literate Haskell conventions
     | Ext_abbreviations       -- ^ PHP markdown extra abbreviation definitions
     | Ext_auto_identifiers    -- ^ Automatic identifiers for headers
+    | Ext_header_attributes   -- ^ Explicit header attributes {#id .class k=v}
     | Ext_implicit_header_references -- ^ Implicit reference links for headers
     deriving (Show, Read, Enum, Eq, Ord, Bounded)
 
@@ -129,6 +130,7 @@ pandocExtensions = Set.fromList
   , Ext_superscript
   , Ext_subscript
   , Ext_auto_identifiers
+  , Ext_header_attributes
   , Ext_implicit_header_references
   ]
 

src/Text/Pandoc/Readers/Markdown.hs

@@ -373,40 +373,48 @@ header :: MarkdownParser (F Blocks)
 header = setextHeader <|> atxHeader <?> "header"
 
 --  returns unique identifier
-addToHeaderList :: F Inlines -> MarkdownParser String
-addToHeaderList text = do
+addToHeaderList :: Attr -> F Inlines -> MarkdownParser Attr
+addToHeaderList (ident,classes,kvs) text = do
   let headerList = B.toList $ runF text defaultParserState
   updateState $ \st -> st{ stateHeaders = headerList : stateHeaders st }
   (do guardEnabled Ext_auto_identifiers
       ids <- stateIdentifiers `fmap` getState
-      let id' = uniqueIdent headerList ids
+      let id' = if null ident
+                   then uniqueIdent headerList ids
+                   else ident
       updateState $ \st -> st{ stateIdentifiers = id' : ids }
-      return id') <|> return ""
+      return (id',classes,kvs)) <|> return ("",classes,kvs)
 
 atxHeader :: MarkdownParser (F Blocks)
 atxHeader = try $ do
   level <- many1 (char '#') >>= return . length
   notFollowedBy (char '.' <|> char ')') -- this would be a list
   skipSpaces
-  text <- trimInlinesF . mconcat <$> manyTill inline atxClosing
-  id' <- addToHeaderList text
-  return $ B.headerWith (id',[],[]) level <$> text
+  text <- trimInlinesF . mconcat <$> many (notFollowedBy atxClosing >> inline)
+  attr <- atxClosing
+  attr' <- addToHeaderList attr text
+  return $ B.headerWith attr' level <$> text
 
-atxClosing :: Parser [Char] st String
-atxClosing = try $ skipMany (char '#') >> blanklines
+atxClosing :: Parser [Char] st Attr
+atxClosing =
+  try $ skipMany (char '#') >> skipSpaces >> option nullAttr attributes <* blanklines
+
+setextHeaderEnd :: MarkdownParser Attr
+setextHeaderEnd = try $ option nullAttr attributes <* blankline
 
 setextHeader :: MarkdownParser (F Blocks)
 setextHeader = try $ do
   -- This lookahead prevents us from wasting time parsing Inlines
   -- unless necessary -- it gives a significant performance boost.
   lookAhead $ anyLine >> many1 (oneOf setextHChars) >> blankline
-  text <- trimInlinesF . mconcat <$> many1Till inline newline
+  text <- trimInlinesF . mconcat <$> many1 (notFollowedBy setextHeaderEnd >> inline)
+  attr <- setextHeaderEnd
   underlineChar <- oneOf setextHChars
   many (char underlineChar)
   blanklines
   let level = (fromMaybe 0 $ findIndex (== underlineChar) setextHChars) + 1
-  id' <- addToHeaderList text
-  return $ B.headerWith (id',[],[]) level <$> text
+  attr' <- addToHeaderList attr text
+  return $ B.headerWith attr' level <$> text
 
 --
 -- hrule block