dig/delv/mdig +yaml output

• YAML output containing multiple messages (i.e., multiple queries and responses) is now implemented as a list, because Paul found that otherwise he couldn't parse more than one object from a stream. dnstap-read -y doesn't do this, which causes me to wonder if it's broken, but nobody's complained.dig +qr will emit both query and response messages. mdig doesn't support +qr so it only emits response messages. delv only supports one QNAME at a time, so it only emits one record, not a list.
• dig +yaml and delv +yaml output are modeled on dnstap-read -y, with a few differences:
  • the identity and version fields come from the frame stream and make no sense here
  • there's no DNSTAP message type (e.g., CQ or RR or whatever)
  • only the query_message_data/response_message_data sections are included, not the query_message or response_message sections, because those are basically identical to the existing output of dig, and I reckon if that's what someone needs, then they can just run dig.

Closes #1145 (closed)