Add plugin code

[Readme](https://github.com/UseAllFive/scrubify.js/blob/plugin/README.md)

[Video mapped to document scroll](http://useallfive.github.io/scrubify.js/document-example.html)

[Video mapped to div scroll](http://useallfive.github.io/scrubify.js/inside-div-example.html)

I'd prefer accuracy/consistency over a negligible performance boost like this. Say `contentBody` has data loaded into it dynamically; the height of `contentBody` will change, but `container` likely wont. 
On the flip side, say you're using the default `$(window)` as the `container` and `$(document)` as the `contentBody`.  Resizing the window doesn't change the height of the document.