Added RTL support

Greetings,

I have checked the previous attempt (https://github.com/kenwheeler/slick/pull/200) to add RTL support to Slick. It was checking for RTL in 17 different places and the approach it followed wasn't the best I believe.

Documents in RTL mode are by default flipped. The only thing needed to enable RTL support on Slick was to change the direction of the animation and the value of translate3d. The value only needed to be flipped (from negative to positive) provided that the document or the parent element of the slider has the `dir="rtl"`attribute.

The modification only check for the rtl in 2 places only.

Some changes were made to the CSS to change the position, direction and the shape of the arrows.

**Note:** The only requirement is to add the attribute `dir="rtl"`to the document or the parent element of the slider.

I have added examples to the page.

Regards
Flipping the value of `targetLeft` in case of `rtl : true;`