Feauture: Thumbnails mode

Simple thumbnails mode.

**Demo**: http://jaaah.github.io/slick/thumbnails.html

![Thumbnails mode](http://i.imgur.com/jtJ9PEf.jpg)

This mode is useful when the Slick used for carousel images.
Thumbs mode similar to dots mode.

Tried to make this scenario with two synced Slick instances, but attempt failed.