writeout: support to generate JSON output

writeout: support to generate JSON output

This commit adds support to generate JSON via the writeout feature:

    -w "%{json}"

It leverages the existing infrastructure as much as possible. Thus,
generating the JSON on STDERR is possible by:

    -w "%{stderr}%{json}"

This implements a variant of
https://github.com/curl/curl/wiki/JSON#--write-out-json.

Closes #4870