configurable skrollrBody

Hi,

since I have different layouts for pages bigger/smaller then 1000px via media-queries I needed a way of manually defining skrollrBody which was hardcoded to `skrollr-body`.

It is totally fine to me if you reject this PR.

If you have other requirements (like an updated Readme.md) I need to fulfil before this can be merged, please feel free to tell me.