Improves configuration migration, and fixes some issues

Fixes incorrect dropItems instead of dropItem key in config.yml
Adds some missing dropper messages to config.yml
Adds proper migration keys for all known configuration options
Adds a modified version of Stargate's StargateYamlConfiguration to allow retaining comments during configuration migration.
Fixes the annoying "[]" is not a valid repairable item
Removes the use of data keys for the global configuration

src/main/java/net/knarcraft/blacksmith/util/ItemHelper.java

@@ -244,7 +244,7 @@ public final class ItemHelper {
         //Parse every material, and add to reforgeAble items
         for (String item : itemList) {
             //Ignore ",,"
-            if (InputParsingHelper.isEmpty(item)) {
+            if (InputParsingHelper.isEmpty(item) || item.matches("\\[]")) {
                 continue;
             }