Allow disabling of CSS transitions

I have a particular use case where I need to outright disable CSS transitions regardless of browser support. This is not currently possible with Slick because it is not exposed as a global. Given my situation, I think it may be useful to provide this as an option, defaulting to `true` where support is available ([compare to FlexSlider](http://www.woothemes.com/flexslider/#gist9481310)).