This patch adds an example of a Next.js app using analytics. A custom
document injects the [Segment](https://segment.com) bootstrapping
snippet into the `<head>`, allowing "page" and "track" calls to be made.
An issue came up in CNA asking how we handle this in our apps (see
https://github.com/segmentio/create-next-app/issues/24), so I figure an
"official example" could help.
NOTE: I am affiliated with Segment.
