Add flag to enable secure SSH features

[normana10]

Long time user

I recently automated some of my rsync-time-backup backups with cron and now I get hourly emails warning me that the host key has been added to the list of known hosts

This is due to the -o UserKnownHostsFile=/dev/null SSH flag being used

I've added a flag --secure-ssh which is disabled by default (so the current default behavior is maintained) but when enabled removes the SSH flag (thus enabling the SSH security features)

I no longer get emails from Cron 👍

This closes #181 and #104

Thanks!