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

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 ` 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.

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.