Can set extra fields requested via Meteor.settings

The current value of `extraFields` remains as a default.

I want this change so that I can set `Meteor.settings.linkedin.extraFields` to

    "email-address,location:(name),picture-url;secure=true,public-profile-url,positions"

because the "positions" field only requires request of the basic profile, whereas "three-current-positions" (which is hard-coded as an extra field in this package) requires request of the full profile.

I currently use `Meteor.settings.linkedin` to set  "clientId" and "secret" during app initialization (after `meteor add service-configuration`),
```
ServiceConfiguration.configurations.remove({
  service: "linkedin"
});
ServiceConfiguration.configurations.insert({
  service: "linkedin",
  clientId: Meteor.settings.linkedin.clientId,
  secret: Meteor.settings.linkedin.secret
});
```
so having `extraFields` accessible as a property of `Meteor.settings.linkedin` would be convenient.