Adds Ability to change slide from an external element

Adds a `slideTriggerSelector` option. This allows you to pass a jQuery selector string, which can trigger a slide change from outside of the 'slick' element.

Example Markup:

```
<div class="slick-container">
  <div>your content</div>
  <div>your content</div>
  <div>your content</div>
</div>

<ul>
  <li><a class="slick-selector" data-slick-target="0" href="#"></a></li>
  <li><a class="slick-selector" data-slick-target="1" href="#"></a></li>
  <li><a class="slick-selector" data-slick-target="2" href="#"></a></li>
</ul>

```

Example JS:
```
$('.slick-container').slick({ slideTriggerSelector: '.slick-selector' })
```

Re-minified using http://jscompress.com/