Download Video (URL Helper)

This little thing will:

• create a System Settings page to enable/disable
• when enabled, cause apps trying to open a link to offer a "Download Video" option
• use youtube-dl to download the video to `~/Videos/youtube-dl`
• once finished, will open the video with the default app for that media type
• it will only only work on URLs supported by youtube-dl, not just any ole website url

NOTE: this will call youtube-dl with the -exec xdg-open file parameter. There may be a security risk involved, if a Bad Person crafts the URL in a way that the resulting file can do Bad Things on your system. Download responsibly.