Add support for CloudFlare's new 1.1.1.1 DNS-over-HTTPS service

CloudFlare [just announced](https://blog.cloudflare.com/announcing-1111/) a new DNS resolver at [1.1.1.1](https://1.1.1.1/), and supports [DNS-over-HTTPS](https://developers.cloudflare.com/1.1.1.1/dns-over-https/).  It would be great to expand the proxy service in CoreDNS to support this, in addition to Google's DNS-over-HTTPS service.