servers:ssh should allow specifying path to SSH key

[kaelansmith]

Current Outcome

Currently the spinupwp servers:ssh command doesn't allow you to specify the path to the SSH private key to use (eg. -i ~/.ssh/spinupwp)

Desired Outcome

Ideally I can run spinupwp servers:ssh <server_id> <user> <private_key_path> (eg. spinupwp servers:ssh 12345 admin ~/.ssh/spinupwp); or perhaps any additional flags get passed through to the underlying ssh command (eg. spinupwp servers:ssh 12345 admin -i ~/.ssh/spinupwp)

If I find time, I can submit a PR; but figured I'd first check to see if this is something you guys are willing to add, or if I'm missing something.