Only prompt for `--auto-sudo` when needed

[lgarron]

Consider the following:

hosts add --auto-sudo 127.0.0.1 cdn.cubing.net
sudo -k # clear `sudo`
hosts add --auto-sudo 127.0.0.1 cdn.cubing.net

Since adding a host is idempotent, the second invocation doesn't change the /etc/hosts file, and hosts can tell this without asking for sudo. It would be nice if --auto-sudo was a little smarter and avoided the prompt. (In fact, that was sort of my assumption based on the name.)