YouTube Video and Audio Downloader (WebEx) nga feller

a rewrite of YouTube video and audio downloader add-on by @inbasic written in WebExtension.

Notes:
1. After installation, go to the options page and
-> 1. (Optional) Install the native client (This is a small NodeJS client written by @andy_portmen)
-> 2. (Optional) Install the FFmpeg executable

2. FFmpeg integration is optional and required only if high resolution video or audio files are preferred (when combining audio-only and video-only streams is needed).

Note: As of version 1.0.0, this extension uses https://github.com/fent/node-ytdl-core as its detection core.