Ability to use a scroll container other than window

[chriseverson]

Currently vue-affix only supports scroll events that take place on the window object. Ideally this could also be applied to other DOM objects when the window isn't the main scrollable container.

A common use case for this functionality would be when implementing something like Slideout.js which disables scrolling on the window, instead replacing it with an interior DOM element.

[eddiemf]

Thanks for the help :)