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.

1 files changed, 3 insertions, 1 deletions

diff --git a/src/Text/Pandoc/Writers/HTML.hs b/src/Text/Pandoc/Writers/HTML.hs
index d8a9c6b81..6222bd810 100644
--- a/src/Text/Pandoc/Writers/HTML.hs
+++ b/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]