v3.2.0: new server placeholder, playersOnline improvements and config backup system

This update brings a few minor improvements to the plugin.

New whitelistEnabled placeholder

The Spigot/Paper version now has a new ${server whitelistEnabled} placeholder. When used will it return true or false based on if the whitelist is enabled.

Multiple worlds/servers for playersOnline

The playersOnline placeholder now allows to set multiple worlds/servers separated by spaces.
When multiple are provided will the placeholder combine each player count and return the total.

Config backups

The plugin will now make a backup of your old config whenever it makes a config migration. This should help restore an old config in case something breaks.
Should the backup fail will the entire config migration be cancelled.

Backups are stored in a dedicated backups folder in the plugin's own directory and will have the format config_<ISO date>.yml