Allow pair connection to my account (rather than tmux)

* By using the --command option, the pair will only have access to the
  tmux session. Since the session is running in my directory anyway,
  this is effectively the same access they'd have by logging into the
  tmux account and accessing a shared session. Hopefully someone will
  call me out if this is terrible and insecure :)
Use my proposed "--lockdown" option for gh-auth

* This option would disable forwarding for the pair's SSH connection.
* See https://github.com/chrishunt/github-auth/pull/25 for more
  information.