Add support for "list" output

[handrews]

There are two new output formats proposed for the next version of JSON Schema, with new names so they can be added alongside the existing ones, which will be deprecated (except for "flag"). There is no technical reason not to support them for 2019-09 and 2020-12 as well.

This issue tracks the "list" format, for which I am likely to submit a PR assuming this is deemed desirable to do now.

[jugaadi]

Config(global and per validation level) to stop processing as soon as an error is encountered would be useful.

[handrews]

@jugaadi this issue is just about formatting the output, so you probably want to file that idea separately.