This allows inserting arbitrary extra arguments into the youtube-dl command-line within each podcast feed entry. This allows, for example, requesting that youtube-dl embed subtitles and captions into the video file, or configuring additional postprocessor arguments.
No effort has been made to make sure no incompatible arguments are provided, with the expectation that the end-user will understand what's possible, and that this is an option expressly for advanced users.
This would fix #162,and theoretically also #136!
