remove --subdirs and --workers args, put in cofig

- this closes issue #17
- also simplified argparse so make it more clear it just gets the
  verbosity level, hopefully this will serve as a reminder next time I
  am included to add an option
- also renamed ['downloads'] --> ['download_directory'] for more clarity
- tweaked formatting of json.dump
now correctly detects already downloaded files

- closes issue #18
- also, clarified/simplified what happens when no suitable downloads were
  found during update
Merge branch 'iss17-ArgsInConfig' into develop
Merge branch 'iss18-FixDetection' into develop
Merge branch 'develop' for v2.3.2