wg-caching status update 2019-09-16

## What was accomplished?

- Convert amppkg from dep to go mod
- Validator verifies that template=”” matches a template id on the page
- Beginning work on ACME cert renewal in amppkg
- Welcome Allan Banaag to https://github.com/ampproject/wg-caching
- `<style amp-custom-length-check>` special error in AMP Validator

This update includes several previous weeks.
