Add rework.inline to encode images to base64

Add `rework.inline(dir, anotherDir)` to inline images and fonts to CSS.

```css
.logo {
  background: inline(icons/logo.png) no-repeat;
}
```

Inline is important to every project, so I think it must be in core.

I add special function `inline(path)` because image optimization need human hands. I can use “inline all” or “inline small” strategy in my projects :). Also same manual way to inline is used in Sass/Compass.