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 <what-goes-here?>
```

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
```
