message: add add_tracks_to_playlist()
diff: @@ -492,6 +492,24 @@ impl Spotify {
  self.delete(&mut url, json!({}));
  }
  + ///https://developer.spotify.com/web-api/add-tracks-to-playlist/
  + ///Adds tracks to a playlist
  + ///Parameters:
  + ///- user_id - the id of the user
  + ///- playlist_id - the id of the playlist
  + ///- track_ids - a list of track URIs, URLs or IDs
  + ///- position - the position to add the tracks
  + pub fn add_tracks_to_playlist(&self,
  + user_id: &str,
  + playlist_id: &mut str,
  + track_ids: Vec<String>,
  + position: Option<i32>) {
  + let plid = self.get_id(Type::Playlist, playlist_id);
  +
  + }
  +
  +
  +
  fn convert_result<'a, T: Deserialize<'a>>(&self, input: &'a str) -> Option<T> {
  match serde_json::from_str::<T>(input) {
  Ok(result) => Some(result),
  