I also had to rename the `envAuth()` in the Cloudflare implementation
to avoid the "redeclared" error
acme/dns_challenge_dnsimple.go:41: envAuth redeclared in this block
previous declaration at acme/dns_challenge_cloudflare.go:154
* new ChallengeProvider with Present and CleanUp methods
* new Challenge type describing `http-01`, `tls-sni-01`, `dns-01`
* new client.SetChallengeProvider to support custom implementations