Support modifiers that can operate on list of targets, rather than seeing only one

[pokey]

The most jq-ish way to do it is to support modifiers that take a pipeline as their parameter, so that they can then see the output of the entire pipeline. The modifier will run the pipeline on its input and then can do something to the list of outputs

But @AndreasArvidsson has a compellingly simple proposal in #757 (comment); worth a discussion