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.
that means that animation will always be with "green" color for 1 second. What do you think about letting user decide which animation (CSS animation) he wants?

I'm thinking about something like:

```js
  $(updatedAtSelector).addClass("is-updated-fragment")
```

And in CSS:

```css
.is-updated-fragment {
  animation-name: fadein;
  animation-duration: .3s;
}

@keyframes fadein {
  0% { opacity: 0 }
  100% { opacity: 1 }
}
```