Adds a MP3 Download button next to the subscribe button, thanks to youtubeinmp3 for their simple download service (http://youtubeinmp3.com/api/). Based off magnus's youtube2mp3 code. Just used something different as it wont open another tab.
< Youtube MP3 Download Buttonについてのフィードバック
I've developed a script using the same site.
Before I started I looked around a lot since the quality of the conversion is really important to me.
Anyway, I ran some tests checking the bitrate of the videos and as they say on their FAQ page they convert to the highest bitrate that the video has. Mostly the videos are in 256 kbps (which is the highest that YT offers), but other times it's at 192 kbps and rarely at 128 kbps (for really low quality videos).
If you don't believe me you can go try for yourself.
I spent like 3 hours just downloading over a hundred videos and checking their bitrate in MPC-HC since I didn't want to solely trust their FAQ (press CTRL + 4 to display the bitrate).
Poor quality conversion
This button takes you to a website that does a very poor quality conversion to mp3. Not worth it. There are better websites and better scripts for them here.