Add ScrollList#scrollToPosition

Add method to ScrollList to enable scrolling to a position within the list, as opposed to just scrolling to the top of an item.

Unit Tests
-------------

- Added tests for new method.

How To +10/QA
-------------

```bash
# skip the following steps if you've already initialized the project
$ git clone git@github.com:WebFilings/wf-js-uicomponents.git
$ cd wf-js-uicomponents

# do not skip these!
$ git fetch && 
git checkout scroll_to_position && 
./init.sh && 
grunt qa
```

- Should see no console errors.
- Open the scroll list example.
- Exercise the API via the dev console. `scrollList` is a global reference to the instance in the page. So simply `scrollList.scrollToPosition(...)`.

@lancefisher-wf 
@robbecker-wf 
@patkujawa-wf