Provide ability to specify parent container

[danhyun]

Firstly thanks for this amazing lib, I was looking for a nice non bootstrap solution and this fit the bill.

As I was implementing this lib I noticed that the change event wasn't being fired and after some debugging I've found that the offsetIndex is being determined based on the element's direct parent offset.

In my structure my header elements are nested two levels deep. It would be as we're calculating the offset for each element if we can take into account all parent elements until we hit the main container.

https://github.com/gajus/contents/blob/master/src/contents.js#L477

That way our offset index will be more reliable.

Thanks again.