title: Add class to non-active sections in FullPage.js
body: <p>For those of you who are familiar with fullpage.js, I'm wondering if you've been able to add classes to non-active sections. Specifically I'm wondering if there's a way to add different classes to sections that come before and after the current active section. </p>
  
  <p>For example, it'd be nice to be able to add the class 'before-current' on all sections before the current slide, and 'after-current' for all others, and add/remove classes as you go through all the sections.</p>