HY-1120: Persist Zoom Level Between Pages in Non-Flow Modes

[JIRA](https://jira.webfilings.com/browse/HY-1120)

Consumers of wf-uicomponents (including Books and possibly SWIFT) would like the ability to have the zoom level stay the same when going from page to page in `peek` mode on mobile, for improved UX.

**Solution**

Add a new component, named `ZoomPersistanceProvider` (please let me know if this is not appropriate naming or if it shouldn't belong in the scroll_list directory). An option, called `persistZoom` was also added to the ScrollList constructor. When the option is set to true, and the ScrollList is not in Flow mode, a `ZoomPersistanceProvider` will be instantiated.

**How To +10/QA**
```bash
# skip the following steps if you've already initialized the project
$ git clone git@github.com:WebFilings/wf-uicomponents.git
$ cd wf-uicomponents

# do not skip these!
$ git fetch && 
git checkoutHY-1120 && 
./init.sh && 
grunt qa
```

* Enter the ScrollList demo and set the mode to Peek
* Zoom in or out.
* Click and drag between items. The scale should stay the same.
* Jump to another page using the input. The scale should stay the same.

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Please review: