add a --force flag to dns_update
whats_next: offer DNSSEC DS parameters rather than the full record and in validation allow for other digests than the one we suggest using

fixes #120 (hopefully), in which Gandi generates a SHA1 digest but we were only checking against a SHA256 digest

Also see http://discourse.mailinabox.email/t/how-to-set-ds-record-for-gandi-net/24/1 in which a user asks about the DS parameters that Gandi asks for.