Adding from into public key

[Rahe]

Hello,

I've tested your project, and this is really great :)

One thing i've discovered while testing other bastions (like https://github.com/ovh/the-bastion) that if you add a from={IP} to the public key when inserting it into authorized_keys, you can limit the allowed IP's that can use this key, or hostnames etc.

I'm using a unique IP/Hostname for connecting to hosts, and specifying it can be very usefull.

I think this can be a great addition to the security when copying/pasting the key into the target servers.

Maybe can this be an option into the configuration file ?