[ ] Contribute crypto/ocsp/client.go to x/crypto
[ ] secrets: add AEAD/AAD method for `associated_data` (base64 encoded)
[ ] echo: nicer HTML error messages (https://github.com/pkg/errors/blob/master/stack.go)
[ ] Use https://github.com/caddyserver/certmagic instead of x/crypto/acme
[ ] Use https://github.com/libdns/libdns to support certmagic