SSH lockdown option

Hey man, I've been reading up on SSH security and considering the implications of allowing remote connections to one's machine. I was pleasantly surprised to find that you've added the `--command` option to the gem, but I thought it might be nice to take it a little further and allow you to specify further "lockdown" of these connections, limiting things such as port forwarding.

I admit knowledge of the subject is limited, but hopefully this can kick off a discussion about making things more secure! :smile: