add the list of opened elements to report

[craff]

This could help with issue #74.

[craff]

We could use report/detailed_report to avoid mapping the list of open_elements at each report if detailed_report is not demanded by the user ?

[craff]

Here is an example of use!

let parse_html ~dynamic ~filename s =
  let detailed_report opens (line,col) err =
    match opens, err with
    | ((_,"ml"), _, _)::_, (`Bad_token _ | `Bad_content _) -> ()
    | _ ->
       Printf.eprintf "File %S, line %d, characters %d:\n %s\n%!"
         filename line col (Error.to_string err);
       exit 1;
  in
  let element = element ~dynamic ~filename in
  let context = if dynamic then Some `Document else None in
  s |> parse_html ~detailed_report ?context |>
    trees_with_loc ~element ~text ~comment ~pi ~xml ~doctype