Simple Economy.format implementation

This change fixes interfacing with some economy-dependant plugins, for instance ShopChest. Related issue: EpicEricEE/ShopChest/issues/359

src/main/java/me/SuperRonanCraft/BetterEconomy/resources/economy/EconomyImplementer.java

@@ -41,7 +41,7 @@ public int fractionalDigits() {
 
     @Override
     public String format(double v) {
-        return null;
+        return String.format("%,.0f", v);
     }
 
     @Override