Add built-in support for JSON(L) mapping

[anjackson]

Rather than having to map JSON outside of the tool, or write a script, it would be possible in principle to provide a 'jq'-style mapping to turn input JSON into the right format. This could use jq directly or use the JMESPath library which is pure Python.

Rather than leaping straight to this, perhaps better to document how to do this kind of thing using jq commands instead?