2020-02-06 offline video downloads from youtube and dozens of other websites

I have long known an open-source project that downloads content from many websites. This update integrates downloading from all its supported video sites into EFV for convenience.

Updates:

• One: Added offline video downloads from dozens of websites, seamlessly integrated into transcoding, using youtube-dl.
• Two: Added a download-center API. Create tasks through the API and build your own frontend for offline video downloading.

Supported sites:

https://ytdl-org.github.io/youtube-dl/supportedsites.html