Right outer joins - report domains in the compared list that are not in the main list.

[StevenBlack]

This arises from StevenBlack/hosts#1235

When comparing two lists we presently show the intersection as a summary or as a list.

It would be nice to extract from the -c list not just the intersection, but what's unique.

[StevenBlack]

Still to do: write a test for this.