Improvement: Make 'initial_replication: all' the default

[ChristophSchmidpeter]

This tool refers itself as 'ZFS replication' tool. As mentioned, creating backups/replicating usually involves backing up everything, so you lose nothing when your source storage fails afterwards. Unfortunately, by default zrepl backs up only the last snapshot, which is arguably unexpected to new users and thus easily can cause a wrong impression of data safety and thus data loss.

Therefore, I suggest making 'initial_replication: all' the default, being an arguably much more common use-case, being more intuitive and being safer in terms of data integrity.