WIP Enhanced Ecommerce integration

Implements Google Analytics Enhanced Ecommerce (Fixes https://github.com/segmentio/analytics.js-integrations/issues/344)

The program requirements:
https://developers.google.com/analytics/solutions/ecommerce-platform#requirements

The original PR which hacks it into existing shop analytics:
https://github.com/Shopify/shopify/pull/25720

Tasks
- [x] Initialize ec.js
- [ ] Product list impressions and clicks (for example, measuring the impression of a product in a list of search results)
- [ ] Internal promotion impressions and clicks (such as banners displayed to promote a sale on another section of a website)
- [ ] Product detail views
- [ ] Add/remove from cart
- [ ] All checkout steps and checkout options where relevant
- [ ] Transactions
- [ ] Specifying local currencies
- [ ] Coupon support
- [ ] Refunds

cc @cjoudrey