Add mostly working postgis example

[jschaf]

This example is broken because there's no Go type for geography. The implementation of geography would look a lot like the existing geographic points in pgtype, like Point.

I'm not going to merge this PR because including Postgis by default is a bit too broad of a change for my liking. Instead, end users should run their own Postgres instances and use the --postgres-connection string:

docker --rm --detach postgis/postgis

pggen gen go \
    --postgres-connection "host=localhost user=postgres dbname=postgres" \
    --schema-glob example/postgis/schema.sql \
    --query-glob example/postgis/query.sql \
    --go-type 'geography=github.com/TODO.Geography'

[jschaf]

Closing. See first comment.

I'm not going to merge this PR because including Postgis by default is a bit too broad of a change for my liking.