Implemented Ext_header_identifiers, Ext_implicit_header_references.

Now by default pandoc will act as if link references have been defined
for all headers.  So, you can do this:

    # My header

    Link to [My header].
    Another link to [it][My header].

Closes #691.

README

@@ -858,7 +858,7 @@ wrapping). Consider, for example:
 
 ### Header identifiers in HTML, LaTeX, and ConTeXt ###
 
-**Extension**
+**Extension: `header_identifiers`**
 
 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.
@@ -906,6 +906,30 @@ and the identifier will be attached to the enclosing `<div>`
 sections to be manipulated using javascript or treated differently in
 CSS.
 
+**Extension: `implicit_header_references`**
+
+Pandoc behaves as if reference links have been defined for each header.
+So, instead of
+
+    [header identifiers](#header-identifiers-in-html)
+
+you can simply write
+
+    [header identifiers]
+
+or
+
+    [header identifiers][]
+
+or
+
+    [the section on header identifiers][header identifiers]
+
+If there are multiple headers with identical text, the corresponding
+reference will link to the first one only, and you will need to use explicit
+links to link to the others, as described above.
+
+Unlike regular reference links, these references are case-sensitive.
 
 Block quotations
 ----------------

src/Text/Pandoc/Options.hs

@@ -89,6 +89,8 @@ data Extension =
     | Ext_hard_line_breaks    -- ^ All newlines become hard line breaks
     | Ext_literate_haskell    -- ^ Enable literate Haskell conventions
     | Ext_abbreviations       -- ^ PHP markdown extra abbreviation definitions
+    | Ext_header_identifiers  -- ^ Automatic identifiers for headers
+    | Ext_implicit_header_references -- ^ Implicit reference links for headers
     deriving (Show, Read, Enum, Eq, Ord, Bounded)
 
 pandocExtensions :: Set Extension
@@ -125,6 +127,8 @@ pandocExtensions = Set.fromList
   , Ext_strikeout
   , Ext_superscript
   , Ext_subscript
+  , Ext_header_identifiers
+  , Ext_implicit_header_references
   ]
 
 strictExtensions :: Set Extension

src/Text/Pandoc/Parsing.hs

@@ -721,6 +721,7 @@ data ParserState = ParserState
       stateAuthors         :: [[Inline]],    -- ^ Authors of document
       stateDate            :: [Inline],      -- ^ Date of document
       stateHeaderTable     :: [HeaderType],  -- ^ Ordered list of header types used
+      stateHeaders         :: [[Inline]],    -- ^ List of headers (used for implicit ref links)
       stateNextExample     :: Int,           -- ^ Number of next example
       stateExamples        :: M.Map String Int, -- ^ Map from example labels to numbers
       stateHasChapters     :: Bool,          -- ^ True if \chapter encountered
@@ -747,6 +748,7 @@ defaultParserState =
                   stateAuthors         = [],
                   stateDate            = [],
                   stateHeaderTable     = [],
+                  stateHeaders         = [],
                   stateNextExample     = 1,
                   stateExamples        = M.empty,
                   stateHasChapters     = False,

src/Text/Pandoc/Writers/HTML.hs

@@ -281,7 +281,9 @@ elementToHtml slideLevel opts (Sec level num id' title' elements) = do
                         then filter isSec elements
                         else elements
   let header'' = if (writerSectionDivs opts ||
-                     writerSlideVariant opts == S5Slides || slide)
+                     writerSlideVariant opts == S5Slides ||
+                     slide ||
+                     not (isEnabled Ext_header_identifiers opts))
                     then header'
                     else header' ! prefixedId opts id'
   let inNl x = mconcat $ nl opts : intersperse (nl opts) x ++ [nl opts]