[ytdl] implement parsing ytdl command-line options (#1680)

- adds 'config-file' and 'cmdline-args' options
  for both ytdl downloader and extractor
- create 'ytdl' helper module, which combines YoutubeDL creation
  and option parsing.
- most likely a buggy mess due to incompatibilities between the
  original youtube-dl and yt-dlp.

CHANGELOG.md

@@ -1,5 +1,7 @@
 # Changelog
 
+## Unreleased
+
 ## 1.19.2 - 2021-11-05
 ### Additions
 - [kemonoparty] add `comments` option ([#1980](https://github.com/mikf/gallery-dl/issues/1980))