Some way to see network activity

There should be some way to have dog emit statements when packets are sent and received, so a user making lots of requests (or a user who is just curious) can see their progress. Something similar to what nping does, possibly ANSI-dimmed so it does not distract from the actual output table.

Currently you can do this by setting DOG_DEBUG to non-empty, but this activates a lot of developer-related output that is not guaranteed to remain stable between releases.

Also related to #99