Accessibility Update

Status: **Opened for visibility**
Reviewers: @jeffkamo @scalvert @tedtate 

## Changes
- Pinny generates a unique id for the header and specifies that id in an `aria-labelledby` attribute on each Pinny
- The modal dialog traps keyboard focus inside the modal dialog so the user can’t accidentally interact with the window behind the modal dialog.
- The position of focus before the modal window opens is saved and the focus is restored to this location after the modal dialog closes.
- Interactivity with the page main content is disabled when Pinny is open.
- We add a `role="document"` attribute to the `pinny__wrapper`.
- After opening pinny, focus is placed on Pinny to start screen readers on that content.

## Notes
- Fixes #30 
- Fixes #31

## How to Test

1. Enable VoiceOver on your Phone or use [ChromeVox](http://www.chromevox.com/) for Chrome.
2. Attempt to use Pinny with a screen reader.
3. Try navigating between form elements when Pinny is open.
So this function should be in Plugin, not here. Let's move it there. Want me to do that?