Sticky scrollbars when container spans out of the viewport

[jfschwarz]

Thanks for this neat little library! We're successfully using it in production at Effektif (http://www.effektif.com).

We use it to display a list that is scrollable horizontally. When the list grows long, the horizontal scrollbar may sit outside of the visible part of the screen. Therefore, we extended your lib so that scrollbars can be configured to be sticky to the bottom of the screen if the scroll container spans out of the viewport. You can see an example on the examples page of our fork (https://github.com/effektif/react-scrollbars).

Would you be interested to include this feature? I would be happy to submit a PR.