-Added player_ip_limitations configuration that prevents stats and rewards gain when 2 players with the same IP are playing.
-Added /minechess challenge command. Now players can challenge other players instead of directly joining an arena.
-Added "challenges" section in the config with 2 configurations: enabled, invitation_expiration_time.
-Added 4 new API Events: ArenaPreJoinEvent, ArenaJoinEvent, ArenaPreLeaveEvent and ArenaLeaveEvent.
-Added scoreboard_enabled option in the config.
-Added the following PlaceholderAPI variables to check elements when the player is inside an arena: %minechess_playerarena_player_white%, %minechess_playerarena_player_black%, %minechess_playerarena_points_white%, %minechess_playerarena_points_black%, %minechess_playerarena_remaining_time_white%, %minechess_playerarena_remaining_time_black%, %minechess_playerarena_status%
-Fixed issues with reward variables. Now you can use %player_loser% and %player_loser_points% variables on: end_by_time, end_by_player_time, end_by_checkmate
