[FEATURE] Extra Option for Backups on Single Installation with Multiple Instances servers

* **Command:** ./gameserver backup
This feature request is for an extra option for ./gameserver backup on servers with Single Installation with Multiple Instances. The option would enable a 1 command backup for the entire game server. With one option, it would stop all instances of the servers, lock them from restarting via ./gameserver monitor/start/restart, as the backup command usually does, and back up each instance noted in the config or all instances created. The second option would, stop all servers, lock them from starting and backup only ONE server instance that is specified in the config file or the instance the command was run from.

The reason for this request is, when you are backing up a single installation with multiple instances type server, you will sometimes encounter a "file was changed as it was read" error if any other instances are running while the backup is being taken. To my knowledge, there is no good way of backing up the server without using a script or external program to manage backups. There is no sense in configuring an external program to do backups when this project has a well thought up backup scheme.