Add a PRE_SAVE_SCRIPT environment variable

It would be nice to have a PRE_SAVE_SCRIPT / PRE_SAVE_SCRIPT_FILE environment variable where the script is executed before "[/save-off](https://github.com/itzg/docker-mc-backup/blob/fed5a0adce71e15380cf9fdb847728da7f983bec/backup-loop.sh#L441)". Most of the time when the server executes a "[/save-all](https://github.com/itzg/docker-mc-backup/blob/fed5a0adce71e15380cf9fdb847728da7f983bec/backup-loop.sh#L446)" it freezes the server for a few seconds. I would be nice to be able to announce to the players, "Server Backup starting in x seconds". Another use cases might be to perform cleanup task like removing drop entities with "/lagg clear", etc... It would be nice to also add a POST_SAVE_SCRIPT but spot is already taken by POST_BACKUP_SCRIPT. I don't know if it would be a breaking change to swap the order of [POST_BACKUP_SCRIPT](https://github.com/itzg/docker-mc-backup/blob/fed5a0adce71e15380cf9fdb847728da7f983bec/backup-loop.sh#L458) and "[/save-on](https://github.com/itzg/docker-mc-backup/blob/fed5a0adce71e15380cf9fdb847728da7f983bec/backup-loop.sh#L455)".

If this gets added the [lines](https://github.com/itzg/docker-mc-backup/blob/master/backup-loop.sh#L435)
```
log INFO "waiting for rcon readiness..."
retry ${RCON_RETRIES} ${RCON_RETRY_INTERVAL} rcon-cli save-on
```
should probably be moved inside the [while loop](https://github.com/itzg/docker-mc-backup/blob/fed5a0adce71e15380cf9fdb847728da7f983bec/backup-loop.sh#L439) to make sure that PRE_SAVE_SCRIPT doesn't get called if the server is offline

Start in backup-loop.sh around the save-off and save-on commands referenced in the issue, then trace the surrounding while loop and existing POST_BACKUP_SCRIPT handling. Define the environment-variable behavior and ordering for PRE_SAVE_SCRIPT and PRE_SAVE_SCRIPT_FILE, including the offline-server case. Done means the pre-save hook runs at the requested point without running when the server is offline.

Written by the indexing model from the issue text.