DRIP: respect explicitly passed campaignId first, then default to settings

discussed this with @yields.

This pull will allow you to explicitly pass a campaignId with an identify call if you choose (to subscribe a user to a specific list). If you don't, the integration will use the one from the settings pane as the default.

adding more documentation for drip, including this behavior, later this afternoon -- see here https://github.com/segmentio/site-docs/issues/309

@calvinfo @amillet89