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 think its safe because worst case scenario the height will remain the same but I do get your point. I am not sure if there is a performance benefit to doing it one way as opposed to another. I will leave this up to you.