userScript: add preview mute functionality

[Informatic]

Closes #18

[fire332]

For the instantaneous animated previews, the b in the screenshot in #18 must be true. You can just mute the previews, but they would never start instantaneously. Also, this looks like a race condition waiting to occur where the video element is animating to the left when the user scrolls right. rect.right should be check as well.

[Informatic]

I don't see any way of having a video reach any border of the screen during preview/scrolling the view, since it's always more-or-less centered, but sure - we can check that as well.

[kopiro]

Can we merge this? I've tested on my device and it also works perfectly :)