Add some way to suppress writing output files

[dempseyatgithub]

Extraction jobs have an output setting of .none which is currently used in testing but is not exposed at all in the command line interface.

Update --output-path to accept the value 'none' to suppress writing output files.

In the unlikely case that there is a need to write files into a directory named 'none' in the current directory, a path that specifies the filename can be used. For example none/proposals.json.