Add per-arena option: display-waves-as-level

If true, this option will cause wave numbers to be displayed as the
players' level to aid in wave tracking. It does NOT replace the
announcements, which can still be disabled by putting blank values
in the announcements-file.

resources/res/settings.yml

@@ -34,3 +34,4 @@ player-time-in-arena: world
 auto-ignite-tnt: false
 auto-start-timer: 0
 use-class-chests: false
+display-waves-as-level: false