Scroll element can be something else than window element

I recently modified headroom to fit my own need, which is the fact that window element is not scrolling in my web app.

Instead, it is another div that has fixed 100% position

I hence introduced a new option, called 'scroller' which accepts a selector for the scrollable element to be defied.

By default, nothing changes and it keeps using the `window` element, otherwise, if the option `scroller` is defined, it looks for the element and bind the scroll event on it.

I though that other people could use this, so I did this PR if you think that way too, you can accept it.

I ran the npm test and updated the README. so everything should be up to date.