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)

Do you think it's safe to assume that `contentBody` will change size with it's container? I don't think we can infer that 100% of the time.