Doesn't work on other elements but body

[naya-u]

Hi!
Can you please add support of optional scrollable element?
I think this can be achieved by simple check if, for example, prop "parentId" is not empty and then if not - use document.getElementById(this.parentId).scrollTop and document.getElementById(this.parentId).offsetHeight instead document.body.scrollTop and document.body.offsetHeight respectively.
I'm not sure if there's gonna be some problems with this solution, but it worked for me.
Huge thanks for plugin, btw!