Command to switch between Krane instances

Implement a command to switch between Krane instances.

Using a sub-command like `context` or `switch` is a possible solution

```
krane switch
```

Another alternative, on a "re-loggin" display a "cached" list of urls to select from...

```
krane login
http://localhost:8500
http://krane.dev:8500
http://biensupernice.com:8500
> Add a new Krane instance
```

Start by examining the CLI entry points for `krane switch` and the existing `krane login` flow. Decide how instances should be identified and whether switching or cached login selection is the intended behavior; done means users can select or switch between multiple Krane instance URLs reliably.

Written by the indexing model from the issue text.