Respect body paddings

### Description
In my setup i have fixed toolbar added by drupal.
It automatically allocate space by adding padding-top to body element.

But fullpage.js calculate section height simply by calling
`$window.height();`

I propose to give users an ability to select another DOM object(body in my case) to calculate height.
This option cannot brake existing setups.

### Steps to reproduce it
1. add padding to body
2. Set `box-sizing: border-box` for the body