YT Better Speedslider por Isaiah

Changes the YouTube 'Custom Speed Slider' to accept up to 4x speed.

1. On a video from YouTube, click on the settings icon (on the bottom right of the video).
2. Click on "Playback Speed", then the "Custom" text at the top of the dialog.
3. The slider values have been replaced. Instead of 1x to 2x, the values are now 1x - 4x.