Commands:
- /referrals - opens GUI
- referrals invite - shows player's unique invite code
- /referrals use <code> - activates friend's unique invite code
- /referrals reload - reloads data and config (only OP)

Placeholders API player:
- %referral_count% - player's total number of succesful referrals
- %referral_count_monthly% - player's referrals by this month
- %referral_code% - player's unique code
- %referral_rank% - player's position of all-time leaderboard
- %referral_rank_monthly% - player's position in monthly leaderboard

Placeholder API leaderboards:
(replace N with number 1-100)
- %referral_leaders_N% - name of player at rank N
- %referral_leaders_count_N% - referral count of the player at rank N
- %referral_leaders_monthly_N% - name of player at rank N (monthly)
- %referral_leaders_monthly_count_N% - referral count of the player at rank N (monthly)

e.g. %referral_leaders_1% - shows first place player's name