Add highlighting

@vast what do you think?

I also want to add some gif in README to illustrate work of `highlight_fragments` and maybe some settings to allow changing highlighting color.
webkit browsers (Opera, Chrome, Safari, Android) still support only prefixed version:

```css

.is-updated-fragment {
  -webkit-animation: fragments-fadein 10s;
  animation: fragments-fadein 10s;
  /* renamed it, so it doesn't interfere with any already defined keyframe with the same name */
}

@-webkit-keyframes fragments-fadein {
  0% { opacity: 0 }
  100% { opacity: 1 }
}

@keyframes fragments-fadein {
  0% { opacity: 0 }
  100% { opacity: 1 }
}
```