[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.

gallery_dl/version.py

@@ -6,4 +6,4 @@
 # it under the terms of the GNU General Public License version 2 as
 # published by the Free Software Foundation.
 
-__version__ = "1.19.2"
+__version__ = "1.19.3-dev"