Refactor config, custom plugin options

[th-ch]

This PR refactors the config:

• simpler API (get/set) for the rest of the code
• allow editing directly the config file
• by doing so, plugins can now have options
• introduce the following options in plugins:
  • adblocker: additional blocklists, caching can be disabled (useful if passing new lists)
  • downloader: custom FFmpeg args (allowing the change the MP3 bitrate for example), custom folder for files