Configurations now properly support lists of serializable objects, and ItemStack is properly serializable. Big thanks to GICodeWarrior for the PR. This fixes BUKKIT-425

By: Nathan Adams <dinnerbone@dinnerbone.com>

paper-api/src/main/java/org/bukkit/configuration/serialization/ConfigurationSerialization.java

@@ -7,6 +7,7 @@ import java.util.HashMap;
 import java.util.Map;
 import java.util.logging.Level;
 import java.util.logging.Logger;
+import org.bukkit.inventory.ItemStack;
 import org.bukkit.util.BlockVector;
 import org.bukkit.util.Vector;
 
@@ -21,6 +22,7 @@ public class ConfigurationSerialization {
     static {
         registerClass(Vector.class);
         registerClass(BlockVector.class);
+        registerClass(ItemStack.class);
     }
 
     protected ConfigurationSerialization(Class<? extends ConfigurationSerializable> clazz) {
@@ -250,4 +252,4 @@ public class ConfigurationSerialization {
 
         return clazz.getName();
     }
-}
+}