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/resources/config.yml

@@ -139,7 +139,7 @@ scrapper:
   defaults:
     
     # Whether the item will drop materials resulting from scrapping on the ground, instead of putting them into the user's inventory
-    dropItems: true
+    dropItem: true
     
     # The chance to fail a salvage, thus destroying the item (0-100)
     failSalvageChance: 0
@@ -175,8 +175,8 @@ scrapper:
       # The message to display when the scrapper is still on a cool-down from the previous salvaging
       coolDownUnexpiredMessage: "&cYou've already had your chance! Give me a break! I'll be ready {time}!"
       
-      # The message to display if the player tries to salvage an item the scrapper cannot salvage
-      cannotSalvageMessage: "&cI'm unable to salvage that item"
+      # The message to display when holding an item the blacksmith is unable to reforge
+      invalidItemMessage: "&cI'm sorry, but I'm a/an {title}, I don't know how to salvage that!"
       
       # The message to display if salvaging the player's item would result in no salvage
       tooDamagedForSalvageMessage: "&cThat item is too damaged to be salvaged into anything useful"
@@ -193,5 +193,8 @@ scrapper:
       # The message to display once the scrapper starts salvaging
       startSalvageMessage: "&eOk, let's see what I can do..."
       
-      # The message to display when holding an item the blacksmith is unable to reforge
-      invalidItemMessage: "&cI'm sorry, but I'm a/an {title}, I don't know how to salvage that!"
+      # The message to display if the player is unable to pay the scrapper's fee
+      insufficientFundsMessage: "&cYou don't have enough money to salvage an item!"
+      
+      # The message to display when explaining the shown item's salvage cost
+      costMessage: "&eIt will cost &a{cost}&e to salvage that item! Click again to salvage!"