Add menu to Mobile Navigation example

[weboverhauls]

Add a dropdown menu to the Mobile Navigation ("Hamburger") menu widget.

The controls current work well, would just be better to have a menu show/hide to give a better real world example.

http://a11y-style-guide.com/style-guide/section-navigation.html#kssref-navigation-navigation-mobile

[cehfisher]

I looked into this enhancement ticket and think this is a good idea, but will require some JS updates or a new demo file so it does not interfere with the actual mobile menu JS on the site. I'll take a stab at it.