Fix behavior of markdown_attribute extension.

It now works as in PHP markdown extra.  Setting `markdown="1"` on
an outer tag affects all contained tags until it is reversed with
`markdown="0"`.  Closes #1378.

Added `stateMarkdownAttribute` to `ParserState`.

src/Text/Pandoc/Parsing.hs

@@ -879,6 +879,7 @@ data ParserState = ParserState
       -- annotate role classes too).
       stateCaption         :: Maybe Inlines, -- ^ Caption in current environment
       stateInHtmlBlock     :: Maybe String,  -- ^ Tag type of HTML block being parsed
+      stateMarkdownAttribute :: Bool,        -- ^ True if in markdown=1 context
       stateWarnings        :: [String]       -- ^ Warnings generated by the parser
     }
 
@@ -958,6 +959,7 @@ defaultParserState =
                   stateRstCustomRoles  = M.empty,
                   stateCaption         = Nothing,
                   stateInHtmlBlock     = Nothing,
+                  stateMarkdownAttribute = False,
                   stateWarnings        = []}
 
 -- | Succeed only if the extension is enabled.

src/Text/Pandoc/Readers/Markdown.hs

@@ -939,10 +939,21 @@ htmlBlock = do
       (TagOpen t attrs) <- lookAhead $ fst <$> htmlTag isBlockTag
       (guard (t `elem` ["pre","style","script"]) >>
           (return . B.rawBlock "html") <$> rawVerbatimBlock)
-        <|> (guardEnabled Ext_markdown_attribute >>
-               case lookup "markdown" attrs of
-                    Just "1" -> rawHtmlBlocks
-                    _        -> htmlBlock')
+        <|> (do guardEnabled Ext_markdown_attribute
+                oldMarkdownAttribute <- stateMarkdownAttribute <$> getState
+                markdownAttribute <-
+                   case lookup "markdown" attrs of
+                        Just "0" -> False <$ updateState (\st -> st{
+                                       stateMarkdownAttribute = False })
+                        Just _   -> True <$ updateState (\st -> st{
+                                       stateMarkdownAttribute = True })
+                        Nothing  -> return oldMarkdownAttribute
+                res <- if markdownAttribute
+                          then rawHtmlBlocks
+                          else htmlBlock'
+                updateState $ \st -> st{ stateMarkdownAttribute =
+                                         oldMarkdownAttribute }
+                return res)
         <|> (guardEnabled Ext_markdown_in_html_blocks >> rawHtmlBlocks))
     <|> htmlBlock'