Add Container Backup support

[rkkoszewski]

It would be great to have support for making Backups like the old LXC Web Panel.

Being able to create a backup and move it to another device, or just be able to rollback a system to a backup can be very useful.

Not sure how the new "lxc-snapshot" feature works, but I doubt it allows to export snapshots as .gz files. So I think backups and snapshots should be separate things.

I might contribute if there is nothing in works, I hope this project is still alive.