`juju switch -` doesn't switch to previous model

[Jack Shaw]

More of a feature request

Lots of cli tools with a navigation aspect use the dash character to navigate back to the previous place. Such as `cd` and `git checkout`

I think it would be great if `juju switch` could do a similar thing!

setup:
```
$ juju models
Controller: lxd

Model Cloud/Region Type Status Machines Units Access Last connection
controller localhost/localhost lxd available 1 1 admin just now
default* localhost/localhost lxd available 3 3 admin 42 seconds ago

$ juju switch controller
lxd:admin/default -> lxd:admin/controller
```

expected (requested) behaviour
```
$ juju switch -
lxd:admin/controller -> lxd:admin/default
```

actual behaviour
```
juju switch -
ERROR model name "admin/-" not valid
```

[Vitaly Antonenko (anvial)]

Thx Jack,

Sounds reasonable.

[Jordan Barrett (barrettj12)]

https://juju.is/docs/olm/juju-stash is related, originally written by Simon IIRC. I'm not sure if it's been tested recently / works with newer versions of Juju.