Add support for using SSH jump/proxy hosts rather than port numbers

[mithro]

• https://wiki.gentoo.org/wiki/SSH_jump_host

This is basically, "SSH into machine A before trying to SSH into machine B".

[mithro]

See also https://goteleport.com/blog/ssh-jump-server/

[mithro]

See also https://zanderwork.com/blog/jump-host/

[CarlFK]

Doesn't this require an account on the jump box?

I have tried to figure out how to create an account that is limited to just forwarding and never felt comfortable with how secure / limited it really was.