Document all scancode toolkit json output keys

[jdaguil]

It's not clear what data keys are available for json output. Currently the output depends on the command line options and even then if data is missing the keys are not shown.

[pombredanne]

We have made some progress towards this: the new #885 code now has some limited schema exposed by every plugin for the top level attributes they contribute to a scan

[pombredanne]

See also #618

[pombredanne]

See also804 #804