LaTeX reader: allow blank space between braced arguments of commands.

For example

    \foo
    {bar}
    {baz}

Closes #2592.

src/Text/Pandoc/Readers/LaTeX.hs

@@ -132,7 +132,9 @@ comment = do
   return ()
 
 bgroup :: LP ()
-bgroup = () <$ char '{'
+bgroup = try $ do
+  skipMany (spaceChar <|> try (newline <* notFollowedBy blankline))
+  () <$ char '{'
      <|> () <$ controlSeq "bgroup"
      <|> () <$ controlSeq "begingroup"