Made some changes to spawner upgrades feature namely:

1. Upgrade prices now increase with the stack size (wasn't the case before)
2. Spawners of the same type & level can be stacked not. Used to only check type before.
3. Added an option of requiring extra spawners in order to upgrade a spawner (feature requested by @Sandi ). Read below for information about this feature & how to enable it.

Spawner Requirement for Upgrading:

• This is a simple incremental requirement needed in order to upgrade your spawners.
• In simple words if you want to upgrade a blaze spawner from level 1 to level 2 you will need 2x level 1 blaze spawners in you inventory while attampting to upgrade.
• Similarly for level 3 you will need 2x level 2 spawners & so on

To enable this feature add the following line in your config.yml:
or alternatively run the command:
/es config default