add get_by_mdns/2

[axelson]

The current does not work with PTR (such as the _elg._tcp.local used by https://github.com/lawik/keylight). This PR adds a new function get_by_mdns that supports looking up PTR records.

Open questions and thoughts:

• The name and docs of this function are inaccurate/confusing because I don't fully understand MDNS, they should be updated before merging
• Should this be combined with gethostbyname somehow? i.e. by expanding gethostbyname to support both types of queries?
• Should this be extended so that it is useful enough for https://github.com/lawik/keylight?
  • Specifically keylight needs at least the port from :srv records: https://github.com/axelson/mdns_lite/blob/0021e43bc8a3e6c304fd85c1664ebadd82141061/lib/mdns_lite/dns.ex#L57-L67
  • But if we include that, perhaps there's additional metadata that is needed as well