Allow developer to specify a scrollable child element

- specifying `childSelector` option overrides default behaviour of
using first child element
- `childSelector` can be a class name (e.g. `.className`) or id (e.g.
`#childID` or `childID`)
- added example of usage to `examples` folder
- this means there is less need to add extra markup to ensure the scrolling element is properly wrapped and is the first element. It allows any element with a selector to be the scrollable element
- Note: only makes first matching class element scrollable