Other providers should be used with the SetChallengeProvider function
and should supply a New function. In your case this would be
NewHTTPProviderWebroot taking the path as an argument.
When using this option, the challenge will be written in a file in
".well-known/acme-challenge/" inside the given webroot folder.
This allows lego to work without binding any port at all.
* 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
