Feature: support multiple client credentials

For those of us who collect email addresses across multiple organizations, it would be great to allow accounts to be assigned to a `client`:

```
gog auth credentials <path1> --client org1
gog auth credentials <path2> --client org2
```

(`--client` defaults to `default`)

Then

```
gog auth add you@gmail.com --client org1
```

...or something. 

