LaTeX reader: be more tolerant of & character.

This allows us to parse unknown tabular environments
as raw LaTeX.  Closes #4208.

src/Text/Pandoc/Readers/LaTeX.hs

@@ -1726,7 +1726,7 @@ inline = (mempty <$ comment)
      <|> (guardEnabled Ext_literate_haskell *> symbol '|' *> doLHSverb)
      <|> (str . (:[]) <$> primEscape)
      <|> regularSymbol
-     <|> (do res <- symbolIn "#^'`\"[]"
+     <|> (do res <- symbolIn "#^'`\"[]&"
              pos <- getPosition
              let s = T.unpack (untoken res)
              report $ ParsingUnescaped s pos

test/command/4208.md

@@ -0,0 +1,18 @@
+```
+% pandoc -t latex
+What is a _piffle_? Mark the correct answer(s):
+
+\begin{TAB}(@)[6pt]{|l|c|}{|c|c|c|}
+(a) a subnormal woffle                      &  $\Box$  \\
+(b) an infinite-dimensional baffle          &  $\Box$  \\
+(c) an inverted first-order triffle         &  $\Box$  \\
+\end{TAB}
+^D
+What is a \emph{piffle}? Mark the correct answer(s):
+
+\begin{TAB}(@)[6pt]{|l|c|}{|c|c|c|}
+(a) a subnormal woffle                      &  $\Box$  \\
+(b) an infinite-dimensional baffle          &  $\Box$  \\
+(c) an inverted first-order triffle         &  $\Box$  \\
+\end{TAB}
+```