Allow customizing of snapshot datetime format

[danyx23]

First of all, thanks for this tool, this is a really convenient way of getting sane backups working quickly :).

I have one suggestion and would send a PR if you agree with the addition: I'd like the datetime format that is used as the directory name of the snapshot to be configurable from the cli. The reason for this is that I am using this on a pretty basic NAS system that exposes the backup folders read only as samba shares, but windows does not allow the colon character in file or directory names that is used as an hour/minutes separator by this script. If this where customizable to e.g. dash then the backups could be directly accessed via SMB. Because this is a NAS system with some weird busybox distribution, this change seems like the most straightforward and maybe others would like to be able to customize the format as well.

If you agree I would add a config option to override the strftime call in

python-rsync-system-backup/rsync_system_backup/__init__.py

Line 613 in 6c7887f

time.strftime('%Y-%m-%d %H:%M:%S'))

Would you be willing to consider such a PR?