Improve argument parsing

[horjulf]

• Use argparse for parsing command line options
• Allow logging level to be defined
  • Use INFO by default
  • Child loggers inherit level from root
  • Add ENV variable support
• Print help if not args specified

usage: scan.py [-h] [--config [CONFIG]] [--logfile [LOGFILE]]
               [--loglevel {WARN,INFO,DEBUG}]
               {sections,server}

Script to assist sonarr/radarr with plex imports. Will only scan the folder 
that has been imported, instead of the whole library section.

positional arguments:
  {sections,server}     "sections": prints plex sections
                        "server": starts the application

optional arguments:
  -h, --help            show this help message and exit
  --config [CONFIG]     Config file location (default: config/config.json)
  --logfile [LOGFILE]   Log file location (default: plex_autoscan.log)
  --loglevel {WARN,INFO,DEBUG}
                        Log level (default: INFO)