Support for fallback gateways

Hi @moul 

I would like to know if it’s possible (or if there are plans) to support **fallback gateways**, meaning:
Try connecting through `bastion1`, and if it fails, automatically retry via `bastion2`.
```yml
...
Gateways:
  - bastion1
  - bastion2
```
Thanks for your work on assh, it's a great tool!