Added optional scrollable viewport other than window

[dopa]

I added the ability to pass in scroll_selector, such as '#people'. Also refactored static "check_binded" variable to be stored as data on the scrollable element.

[mlisowsk]

Hi @dopa I am using your proposed code in my fork and found a bug:
You need to declare the opts var in the top scope otherwise calling $.force_appear() will throw an exception for an undefined variable opts. I use var opts = defaults = ... to avoid exceptions in case $.force_appear() is called before any $.appear() call.