This commit adds a new DNS provider for
[acme-dns](https://github.com/joohoi/acme-dns) to allow Lego to set
DNS-01 challenge response TXT with an ACME-DNS server automatically.
ACME-DNS allows ceding minimal zone editing permissions to the ACME
client and can be useful when the primary DNS provider for the zone does
not allow scripting/API access but can set a CNAME to an ACME-DNS
server.
Lower level ACME-DNS API calls & account loading/storing is handled by
the `github.com/cpu/goacmedns` library.
The provider loads existing ACME-DNS accounts from the specified JSON
file on disk. Any accounts the provider registers on behalf of the user
will also be saved to this JSON file.
When required, the provider handles registering accounts with the
ACME-DNS server domains that do not already have an ACME-DNS account.
This will halt issuance with an error prompting the user to set the
one-time manual CNAME required to delegate the DNS-01 challenge record
to the ACME-DNS server. Subsequent runs will use the account from disk
and assume the CNAME is in-place.
