LaTeX reader: allow non-empty colsep in tables

The `tabular` environment allows non-empty column separators
with the "@{...}" syntax. Previously, pandoc would fail to
parse tables if a non-empty colsep was present. With this
commit, these separators are still ignored, but the table gets
parsed. A test case is included.

src/Text/Pandoc/Readers/LaTeX.hs

@@ -1275,7 +1275,7 @@ complexNatbibCitation mode = try $ do
 parseAligns :: LP [Alignment]
 parseAligns = try $ do
   char '{'
-  let maybeBar = skipMany $ sp <|> () <$ char '|' <|> () <$ try (string "@{}")
+  let maybeBar = skipMany $ sp <|> () <$ char '|' <|> () <$ (char '@' >> braced)
   maybeBar
   let cAlign = AlignCenter <$ char 'c'
   let lAlign = AlignLeft <$ char 'l'

tests/Tests/Readers/LaTeX.hs

@@ -91,6 +91,11 @@ tests = [ testGroup "basic"
           , "Table with empty column separators" =:
             "\\begin{tabular}{@{}r@{}l}One & Two\\\\ \\end{tabular}" =?>
             simpleTable' [AlignRight,AlignLeft] [[plain "One", plain "Two"]]
+          , "Table with custom column separators" =:
+            unlines [ "\\begin{tabular}{@{($\\to$)}r@{\\hspace{2cm}}l}"
+                    , "One&Two\\\\"
+                    , "\\end{tabular}" ] =?>
+            simpleTable' [AlignRight,AlignLeft] [[plain "One", plain "Two"]]
           , "Table with vertical alignment argument" =:
             "\\begin{tabular}[t]{r|r}One & Two\\\\ \\end{tabular}" =?>
             simpleTable' [AlignRight,AlignRight] [[plain "One", plain "Two"]]