LaTeX reader: allow for []s inside LaTeX optional args.

Fixes cases like:

    \begin{center}
    \begin{tikzpicture}[baseline={([yshift=+-.5ex]current bounding box.center)}, level distance=24pt]
    \Tree [.{S} [.NP John\index{i} ] [.VP [.V likes ] [.NP himself\index{i,*j} ]]]
    \end{tikzpicture}
    \end{center}

src/Text/Pandoc/Readers/LaTeX.hs

@@ -861,7 +861,8 @@ opt = bracketed inline
 
 rawopt :: LP String
 rawopt = do
-  contents <- bracketed (many1 (noneOf "]") <|> try (string "\\]"))
+  contents <- bracketed (many1 (noneOf "[]") <|> try (string "\\]") <|>
+                   try (string "\\[") <|> rawopt)
   optional sp
   return $ "[" ++ contents ++ "]"