Allow multicols env in LaTeX reader

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

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" $ spaces *> braced *> spaces *> blocks)
    , ("minipage", env "minipage" $
           skipopts *> spaces *> optional braced *> spaces *> blocks)
    , ("figure", env "figure" $ skipopts *> figure)

test/command/latex-multicols.md

@@ -0,0 +1,11 @@
+# `\begin{multicols}`
+
+```
+% pandoc -f latex -t native
+\begin{multicols}{2}
+Hello
+\end{multicols}
+^D
+[Para [Str "Hello"]]
+```
+