Support gtag.js (Universal Analytics)

## Description

Rewrite this package to support `gtag.js` [(Universal Analytics)](https://developers.google.com/analytics/devguides/collection/gtagjs).

## Implementation model

- Add new `<AnalyticsScript />` which contains our the `gtag.js` and the `gtag()` function.
  - Add `anonymizeIp` and `respectDNT` props to enable these options
- Rewrite event utils to not rely on `react-ga`.
