Stop hidden anchors from being keyboard accessible

As these have been given 'aria-hidden' to hide them from screen readers
we should also ensure that they cannot be tabbed to.

By adding 'tabindex=-1' we ensure that a screen reader user does not tab
to these and get no response.

Fixes #126

lib/assets/javascripts/_modules/anchored-headings.js

@@ -11,7 +11,7 @@
       var $this = $(this)
       $this.addClass('anchored-heading')
       $this.prepend(
-        '<a href="#' + $this.attr('id') + '" class="anchored-heading__icon" aria-hidden="true"></a>'
+        '<a href="#' + $this.attr('id') + '" class="anchored-heading__icon" aria-hidden="true" tabindex="-1"></a>'
       )
     }
   }