LaTeX reader: Fixed subtle bug in tokenizer.

Material following `^^` was dropped if it wasn't a character
escape.  This only affected invalid LaTeX, so we didn't see it
in the wild, but it appeared in a QuickCheck test failure
https://travis-ci.org/jgm/pandoc/jobs/319812224

src/Text/Pandoc/Readers/LaTeX.hs

@@ -376,8 +376,9 @@ totoks pos t =
                          | d < '\128' ->
                                   Tok pos Esc1 (T.pack ['^','^',d])
                                   : totoks (incSourceColumn pos 3) rest''
-                       _ -> [Tok pos Symbol ("^"),
-                             Tok (incSourceColumn pos 1) Symbol ("^")]
+                       _ -> Tok pos Symbol ("^") :
+                            Tok (incSourceColumn pos 1) Symbol ("^") :
+                            totoks (incSourceColumn pos 2) rest'
                 _ -> Tok pos Symbol ("^")
                      : totoks (incSourceColumn pos 1) rest
          | otherwise ->