ACME client: http-01 challenge

```
tls.loader.acme local_tls {
hostname $(hostname)
challenge http-01
}

tls &local_tls # set default TLS config for all endpoints

# Configure reverse-proxy on port 80 to forward ACME challenges here
http tcp://127.0.0.1:8080 {
serve acme_http_01 &local_tls # or just 'serve &local_tls'
}
```

- [ ] HTTP endpoint module
- [ ] http-01 challenge

Forked from #3.