more methods and options

[xMokAx]

1. added analytics.set() method which calls ReactGA.set() with a required argument fieldsObject and optional argument trackerNames.
2. analytics.init(): modified the first argument to be configsOrTrackingId which is the trackingId as a string or array of multiple trackers objects and added initOptions object as a second optional argument.
3. modified ReactGA.event(): now it accepts only one argument options object that requires at least category and action as properties.

[sergiodxa]

The Next.js team has an official plugin being developed, while plugin supports it's still in canary only, the way the plugin works it's simpler than next-ga and I recommend you to check it and do it manually in your project until it's stable.

Plugin link: https://github.com/zeit/next.js/tree/canary/packages/next-plugin-google-analytics