Allow multicols env in LaTeX reader

The multicols environment has a mandatory argument - number of columns.
It is incorrectly parsed as plain string.

Wrap multicols content into nested divs.

Signed-off-by: Igor Pashev <pashev.igor@gmail.com>

Author: ip1981

src/Text/Pandoc/Readers/LaTeX.hs

@@ -1913,6 +1913,7 @@ environments = M.fromList
    , ("abstract", mempty <$ (env "abstract" blocks >>= addMeta "abstract"))
    , ("sloppypar", env "sloppypar" $ blocks)
    , ("letter", env "letter" letterContents)
+   , ("multicols", env "multicols" multicols)
    , ("minipage", env "minipage" $
           skipopts *> spaces *> optional braced *> spaces *> blocks)
    , ("figure", env "figure" $ skipopts *> figure)
@@ -2079,6 +2080,18 @@ letterContents = do
                   _ -> mempty
   return $ addr <> bs -- sig added by \closing
 
+multicols :: PandocMonad m => LP m Blocks
+multicols = do
+  spaces
+  n <- fromMaybe 1 . safeRead . untokenize <$> braced
+  spaces
+  bs <- blocks
+  return $ divWith ("", ["columns"], []) $ cols n bs
+  where
+    cols :: Int -> Blocks -> Blocks
+    cols n = foldr1 (.) (replicate n $ divWith ("", ["column"], []))
+
+
 figure :: PandocMonad m => LP m Blocks
 figure = try $ do
   resetCaption

test/command/latex-multicols.md

@@ -0,0 +1,14 @@
+# `\begin{multicols}`
+
+```
+% pandoc -f latex -t native
+\begin{multicols}{2}
+Hello
+\end{multicols}
+^D
+[Div ("",["columns"],[])
+ [Div ("",["column"],[])
+  [Div ("",["column"],[])
+   [Para [Str "Hello"]]]]]
+```
+