Improve dnst update user interface and functionality?

[mozzieongit]

While reviewing #10 I mentioned that:

1. we should improve on the interface a bit (the "none" in place of an IP address for deletion is bugging me in particular)
2. that it might be useful for dnst update to be able to update more than just A/AAAA records, like TXT records (e.g. for acme challenges).

An initial idea would be the following interface variants:

Variant A

Synopsis:

• dnst update [options] add <domain name> <RRTYPE> <RRs>...
• dnst update [options] delete <domain name> <RRTYPE> [<RRs>...]
• dnst update [options] clear <domain name> (this distinction is to avoid accidental deletion of whole domain names, but might be unnecessary)

Examples:

• dnst update add <domain name> AAAA "::1" "::2"
  • Add multiple AAAA records
• dnst update add <domain name> TXT "challenge"
  • Add multiple TXT records
• dnst update delete <domain name> AAAA ::1 ::2
  • Delete exact AAAA RRs on domain name (::1 and ::2 in this case)
• dnst update delete <domain name> AAAA
  • Delete all AAAA RRs on domain name
• dnst update clear <domain name>
  • Delete all RRSETs on domain name, aka delete the whole domain name

Variant B

(like above, but as options for different RR types at once)

Alternatively prove RRs as options, to be able to update multiple different RR types at once:

• dnst update add <domain name> --a 1.1.1.1 --aaaa ::1 --txt "hello world"
  • Add all these records with the TTL determined from the SOA RR
• dnst update delete <domain name> --a 1.1.1.1 --aaaa ::1 ...
  • Delete these exact records if they exist
• dnst update clear <domain name> --a --aaaa
  • (differs from (A): Delete specific RRSETs on domain name

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The update [options] could then set the --ttl and maybe even --class (but that is probably rarely helpful).

I like variant (A) more.