add variantToApiToken

Would it be useful to be able to map variants to different api tokens?  I would like to upload different variants to separate instances of hockeyapp but the api token generation in hockeyapp only allows access to either one app or all apps.

So we could do something like this:
```
hockeyapp {
  ...
  variantToApiToken = [
    dev: <dev_api_token>
    release: <release_api_token>
  ]
  ...
}
```

I figured I would attempt to implement it myself instead of just asking for you to add the feature this time.