Add diminished returns when a player has earned too much XP

When a player reaches a certain (configurable) threshold value of total
XP earned in a specific skill, his earned XP in this skill will be
decreased.

Depending on how far the player has exceeded the threshold value, his
XP will decrease more. After the (configurable) time interval of 10
minutes, the registered data will be cleared and the player can earn XP
as normal again.

src/main/resources/experience.yml

@@ -59,6 +59,15 @@ Experience_Formula:
     Custom_XP_Perk:
         Boost: 1.25
 
+#
+# Settings for Deminished Returns
+###
+Deminished_Returns:
+    # Limit the amount of experience a player can earn:
+    # Threshold (amount of experience) per Time_Interval (in minutes)
+    Threshold: 20000
+    Time_Interval: 10
+
 #
 #  Settings for XP conversion with '/mcconvert experience'
 ###