Support using private ip of an instance without passing the private ip as an argument

[colwynmyself]

Right now there are two ways to SSH to the private ip of an instance rather than the public ip:

1. Don't have a public ip
2. Pass in the private ip of the instance with a -t <instance-id> argument attached

It would be great if we could pass in an argument like --private to mssh that would make the resulting ssh command use the private ip without having to go fetch the private ip of the instance.

I threw together a quick branch on a fork of the project that supports this, but it doesn't include any tests. master...colwynmyself:support_private_flag

If you'd like I can look into tidying that up a bit or I could even just open a PR against this project if y'all would like.

[scampuza]

@colwynmyself Were you able to connect to the EC2 instance using the Private IP ?

[colwynmyself]

@scampuza When I initially created this PR, yeah. I haven't tested it in over a year but I imagine it would still work since the changes are so simple.

[virmantas-simplex]

@colwynmyself Were you able to connect to the EC2 instance using the Private IP ?

I have instance with Public address (used only for HTTPS connections from the Internet) and Private address (used form ssh into this instance from bastion). When I try to mssh in into this instance from bastion I see mssh is trying to establish connection with external IP.

[gwittel]

Just tested this patch with a fresh install and it works fine. It would be really helpful to have this baked in.