Added a listener for using an offset for fixed navbars

[pimwillems]

Reason for modification

We've added an offset variable to use if you have a fixed navbar. Otherwise the active indicator responds the page top which is underneath the fixed navbar.

How to use / Code example

You can use fixedNavbarOffset to indicate an amount of pixels calculated from the top.

var spy = new ScrollSpy('#js-scrollspy', {
    nav: '.js-scrollspy-nav > a',
    className: 'is-inview',
    fixedNavbarOffset: 170,
    callback: function () {
      $(".name-navigation a").blur();
    }
  });