Add new `fromjsonarray` operation.

This allows importing records contained within JSON arrays.

I found this to be useful in combination with some of the other `recs-*` tools. The intent of this command for `recs-fromjson -k f1,f2,f3` is to work quite similar to how `jq -c '.[]|{"f1": .f1, "f2": .f2, "f3": .f3}'`

See http://stedolan.github.io/jq/ for details.

The usage says that it supports key specs, but I wasn't quite sure how to make that happen. I figure you could use this as a first draft of this feature if nothing else.