docs(download): examples for downloading videos

examples/download/basic/index.js

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+// import Innertube from 'youtubei.js';
+const { createWriteStream } = require('fs');
+const Innertube = require('../../../lib/Innertube');
+
+(async () => {
+    // instantiate the library
+    const yt = await new Innertube();
+
+    // download the video
+    // the default options are to download with 360p quality
+    // with both audio and video in an mp4 container
+    yt.download('bUHZ2k9DYHY')
+        .pipe(createWriteStream('./stream.mp4'))
+        .on('progress', progress => {
+            console.log(`Downloaded ${progress.percent}%`);
+        })
+        .on('finish', () => {
+            console.log('Download finished');
+        });
+})();