Added Ext_abbrevations for PHP markdown style abbreviation keys.

Note:  pandoc does not have an abbreviation element (yet) and
so currently when this extension is enabled, it just causes
pandoc to skip the abbrevation keys.

README

@@ -2243,6 +2243,15 @@ pandoc. If `pandoc_title_block` is enabled, it will take precedence over
 
   [MultiMarkdown]: http://fletcherpenney.net/multimarkdown/
 
+**Extension: `abbrevations`**\
+Parses PHP Markdown Extra abbreviation keys, like
+
+    *[HTML]: Hyper Text Markup Language
+
+Note that the pandoc document model does not support
+abbreviations, so if this extension is enabled, abbreviation keys are
+simply skipped (as opposed to being parsed as paragraphs).
+
 Producing slide shows with Pandoc
 =================================
 

src/Text/Pandoc/Options.hs

@@ -85,6 +85,7 @@ data Extension =
     | Ext_subscript           -- ^ Subscript using ~this~ syntax
     | Ext_hard_line_breaks    -- ^ All newlines become hard line breaks
     | Ext_literate_haskell    -- ^ Enable literate Haskell conventions
+    | Ext_abbreviations       -- ^ PHP markdown extra abbreviation definitions
     deriving (Show, Read, Enum, Eq, Ord, Bounded)
 
 pandocExtensions :: Set Extension

src/Text/Pandoc/Readers/Markdown.hs

@@ -219,10 +219,6 @@ parseMarkdown = do
          $ B.setDate (runF date st)
          $ B.doc $ runF blocks st
 
---
--- initial pass for references and notes
---
-
 referenceKey :: Parser [Char] ParserState (F Blocks)
 referenceKey = try $ do
   skipNonindentSpaces
@@ -256,6 +252,20 @@ referenceTitle = try $ do
                                       notFollowedBy (noneOf ")\n")))
   return $ fromEntities tit
 
+-- | PHP Markdown Extra style abbreviation key.  Currently
+-- we just skip them, since Pandoc doesn't have an element for
+-- an abbreviation.
+abbrevKey :: Parser [Char] ParserState (F Blocks)
+abbrevKey = do
+  guardEnabled Ext_abbreviations
+  try $ do
+    char '*'
+    reference
+    char ':'
+    skipMany (satisfy (/= '\n'))
+    blanklines
+    return $ return mempty
+
 noteMarker :: Parser [Char] ParserState String
 noteMarker = string "[^" >> many1Till (satisfy $ not . isBlank) (char ']')
 
@@ -311,6 +321,7 @@ block = choice [ codeBlockDelimited
                , definitionList
                , noteBlock
                , referenceKey
+               , abbrevKey
                , para
                , plain
                ] <?> "block"