Add option to set item amount in menu

[Lemvs]

• Added the ability to set item amounts in the menu. In the default configuration, I used this in the spawner-amounts menu to show the amount of deposited/withdrawn spawners.
• Changed the check for whether an item's configuration contains a given option from contains() to a function appropriate for the option type.
• Added a catch for incorrectly added flags in the item's configuration.
• Changed the display of item configuration errors in console to be more universal.