Added '/duel accept' Command and Public Duel Announcements
Duel accept command now allows players to accept their most recent duel request. /duel <playername> has been reconfigured to accept a specific duel request instead of just the latest.

Admins can now add announcements in chat before or after a duel. Pre-duel announcements announce who's dueling and allow players to spectate! Post-duel announcements broadcast who won and who lost.

New Features:

• Added configurable announcements before and after a duel that allow players to spectate or view the result! Enjoy community fun!
• Added announce_duels config.yml option (Default: false)
• Added /duel accept
• Added duel_start_announcment config.yml option (Default: "&c%player_one%&6 is dueling &c%player_two%&6! %button%&6&lSpectate here.%button%")
• Added duel_end_announcement config.yml option. (Default:
  "&6Congratulate &c%winner%&6! They just won a match against &c%loser%&6!")

Bugfixes:

• Fixed bug where duel requests persist even after accepting or denying request.
• Fixed bug where a player can spectate another duel while actively dueling
• Fixed bug where players cannot start duel for a few seconds after losing one.
• Fixed bug where players can spam duel requests.