Added OfflinePlayer#balance kotlin extensions

eco-api/src/main/kotlin/com/willfp/eco/core/integrations/economy/EconomyHelpers.kt

@@ -0,0 +1,20 @@
+@file:JvmName("EconomyExtensions")
+
+package com.willfp.eco.core.integrations.economy
+
+import org.bukkit.OfflinePlayer
+
+/**
+ * @see EconomyManager
+ */
+var OfflinePlayer.balance: Double
+    get() = EconomyManager.getBalance(this)
+    set(value) {
+        val diff = this.balance - value
+
+        if (diff > 0) {
+            EconomyManager.removeMoney(this, diff)
+        } else if (diff < 0) {
+            EconomyManager.giveMoney(this, -diff)
+        }
+    }