Allows you to configure which element to append the reveal

  - Checks for the existance of rootElement in the settings object
  - Uses rootElement if it is set, otherwise it appends the reveal
    to <body>

**Depends on PR:** #3743

Usage
----------

To change the element, pass in an a settings object on initialization:
```JS
$(document).foundation( 'reveal', { rootElement: '#alternate-element'})
```
Or on open:
```JS
$('#myModal').foundation( 'reveal', 'open', { rootElement: '#alternate-element' });
```
Purpose
------------
In a QUnit test environment, I needed the reveal to stay within the
confines of the testing sandbox, so that the app code under test
retained access to the modal.

This feature is transparent, if no rootElement is specified, then
reveal behaves exactly as expected. But, if you need to attach the
reveal to another element, then you can.
Merge pull request #3881 from astonishdesign/feature/reveal_root_element_config

Allows you to configure which element to append the reveal