Option to disable drag scrolling

This adds the `touchScrollingEnabled` option for `ScrollList`. When enabled, you can scroll and pan with dragging and swiping. When disabled, you can't.

How To +10/QA
-------------

```bash
git fetch && git checkout option_to_disable_drag_scrolling
./init.sh && grunt qa
#All tests should pass.
grunt serve
```

* Open [http://0.0.0.0:9000/examples/scroll_list/?scroll=flow&touchScrollingEnabled=true](http://0.0.0.0:9000/examples/scroll_list/?scroll=flow&touchScrollingEnabled=true)
* You should be able to drag to scroll the document.
* Now, open [http://0.0.0.0:9000/examples/scroll_list/?scroll=flow&touchScrollingEnabled=false](http://0.0.0.0:9000/examples/scroll_list/?scroll=flow&touchScrollingEnabled=false)
* When you drag, nothing happens.

Might also want to just do a la `options = options || {};` at the top of the ctor for the next option that comes along. Or, something like this: https://github.com/WebFilings/wf-uicomponents/blob/master/src/scroll_list/ScrollList.js#L299