Adding callbacks

Added support for method callback to isInViewport(). Refactored the js so that it will properly handle a collection of elements and apply the callback to each element if needed. Maintained original functionality, so that :in-viewport and isInViewport({}) with a single object option still works. I updated the docs, created a new minified version and example page, and bumped the version to 1.2.0, since this was more than just a minor update.