Attach to existing remote sessions

[wagoodman]

It would be nice to have something that acted like ssh ... -t tmux attach-session or ssh ... -t -- bash -c "tmux attach || tmux new" to attach or create a session. Currently bridgy does:

(TMUX on my local machine)
 |
 + session 1: someotherhost
    \_ window1: ssh user@someotherhost
    \_ window2: ssh user@someotherhost
    \_ ...

but attaching or creating would allow:

(TMUX on my local machine)
 |
 + session 1 linked to an existing session on someotherhost
    \_ window1: shell on someotherhost
    \_ window2: shell on someotherhost
    \_ ...

Note: this needs to be resilient to tmux not being installed on the remote host.