This change extends the Client to allow users request certificate issuance for IP addresses. See the ACME spec extension for details about IP address identifiers: https://tools.ietf.org/html/draft-ietf-acme-ip. Change-Id: I92a8d8fae048487168906c14892c6dd33af10c07 Reviewed-on: https://go-review.googlesource.com/c/160197 Reviewed-by: Filippo Valsorda <filippo@golang.org> Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>