LaTeX reader: Allow skipping of unknown block commands in \author section.

Closes #505, which was a problem with `\vspace{10pt}` inside `\author`.

src/Text/Pandoc/Readers/LaTeX.hs

@@ -298,7 +298,9 @@ authors :: LP ()
 authors = try $ do
   char '{'
   let oneAuthor = mconcat <$>
-       many1 (notFollowedBy' (controlSeq "and") >> inline)
+       many1 (notFollowedBy' (controlSeq "and") >>
+               (inline <|> mempty <$ blockCommand))
+               -- skip e.g. \vspace{10pt}
   auths <- sepBy oneAuthor (controlSeq "and")
   char '}'
   updateState (\s -> s { stateAuthors = map (normalizeSpaces . toList) auths })