Renew certificates using ACME

[twifkak]

Add configuration parameters similar to webpkgserver to support ACME renewal of a certificate. It's not necessary to support all verification methods (DNS/HTTP/ALPN); one is sufficient (whichever is automatable).

This should include some support for monitoring that the certificate is still valid. Here are some ideas:

1. Return an HTTP error (or JS exception?) when the certificate is expired, so it can show up in Cloudflare analytics.
2. Document some curl | openssl command the user could run as a cron job.
3. Let the user configure a webhook URL to be pinged when the certificate is expired.

[twifkak]

@antiphoton added initial support in #159. Remaining work:

• Add a handler for the acme challenge request. It should handle the challenge.
• Document how to use it at credentials/README.md.
• Add a scheduled renewal job.