Further optimization and new options

Still 3kb of minified size (-0.5kb to previous pull request) and 1kb gzipped.

Options added:

* `from` - number of slide to start from. First slide is 1 (not 0), second is 2 (not 1) and so on.
* `use` - thumbs list selector (widely used option). Handles any selectors with `<li>` inside, can be used in context like `'.thumbs'` or `$('.thumbs')`. Adds `active` class for current thumb. Try it:

```html
<ul class="thumbs">
  <li>1</li>
  <li>2</li>
  <li>3</li>
</ul>
```