Org reader: support new syntax for export blocks

Org-mode version 9 usees a new syntax for export blocks.  Instead of
`#+BEGIN_<FORMAT>`, where `<FORMAT>` is the format of the block's
content, the new format uses `#+BEGIN_export <FORMAT>` instead.  Both
types are supported.

src/Text/Pandoc/Readers/Org/Blocks.hs

@@ -168,6 +168,7 @@ orgBlock = try $ do
   blkType <- blockHeaderStart
   ($ blkType) $
     case blkType of
+      "export"  -> exportBlock
       "comment" -> rawBlockLines (const mempty)
       "html"    -> rawBlockLines (return . (B.rawBlock blkType))
       "latex"   -> rawBlockLines (return . (B.rawBlock blkType))
@@ -239,6 +240,14 @@ rawBlockContent blockType = try $ do
 ignHeaders :: OrgParser ()
 ignHeaders = (() <$ newline) <|> (() <$ anyLine)
 
+-- | Read a block containing code intended for export in specific backends
+-- only.
+exportBlock :: String -> OrgParser (F Blocks)
+exportBlock blockType = try $ do
+  exportType <- skipSpaces *> orgArgWord <* ignHeaders
+  contents   <- rawBlockContent blockType
+  returnF (B.rawBlock (map toLower exportType) contents)
+
 verseBlock :: String -> OrgParser (F Blocks)
 verseBlock blockType = try $ do
   ignHeaders

tests/Tests/Readers/Org.hs

@@ -1293,7 +1293,7 @@ tests =
               ]
           ]
 
-      , "Verse block with newlines" =:
+      , "Verse block with blank lines" =:
           unlines [ "#+BEGIN_VERSE"
                   , "foo"
                   , ""
@@ -1302,6 +1302,20 @@ tests =
                   ] =?>
           para ("foo" <> linebreak <> linebreak <> "bar")
 
+      , "Raw block LaTeX" =:
+          unlines [ "#+BEGIN_LaTeX"
+                  , "The category $\\cat{Set}$ is adhesive."
+                  , "#+END_LaTeX"
+                  ] =?>
+          rawBlock "latex" "The category $\\cat{Set}$ is adhesive.\n"
+
+      , "Export block HTML" =:
+          unlines [ "#+BEGIN_export html"
+                  , "<samp>Hello, World!</samp>"
+                  , "#+END_export"
+                  ] =?>
+          rawBlock "html" "<samp>Hello, World!</samp>\n"
+
       , "LaTeX fragment" =:
           unlines [ "\\begin{equation}"
                   , "X_i = \\begin{cases}"