Support configuring new auth context without interactivity

In some cases, it can be useful to configure `doctl` non-interactively. Currently, we support this when setting up the default context. For example, running the following will validate the token and add it to the `doctl` config file without additional user interaction:

```
doctl auth init -t $DO_TOKEN
Using token [asupersekrettokenthatisverylongidontwanttotypeit]

Validating token... OK
```

If the `--context` flag is added to that command, the token is ignored and the user is prompted to enter the token manually:

```
doctl auth init --context new-context -t $DO_TOKEN
DigitalOcean access token:
```

The behaviour should be consistent. When passing a token and a context together to `doctl auth init`, we should validate the token and configure a new context to use it without additional user input.