The config:

#Killstreaks setup
#Use %player% if you want to use a command on the player with the killstreak
Killstreaks:
2: "say wow! You have reached a 2 killstreak!"
5: "give %player% wooden_planks 5"
10: "gamemode creative %player%"
15: "say 15kills"
20: "Say 20kills"
#The above are just some example commands!

#Make sure players are not killing only the same person and abuse the killstreak rewards
Antikillstreakabuse: false