helpers: comparison: add `ifNth` helper

Basically a modulo, useful inside `#each` with `@index`.

Example usage: handling clearfixes in responsive bootstrap grids

```html
<div id="results" class="row">
  {{#each results}}

  <!-- 2 per row; 3 on small; 4 on medium and larger -->
  <div class="col-xs-6 col-sm-4 col-md-3">
    {{foo}}
  </div>

  <!-- make clearfixes for 3 kinds of grids -->
  {{#ifNth "2" @index}}
   <div class="clearfix visible-xs"></div>
  {{/ifNth}}
  
  {{#ifNth "3" @index}}
   <div class="clearfix visible-sm"></div>
  {{/ifNth}}
  
  {{#ifNth "4" @index}}
   <div class="clearfix visible-md visible-lg"></div>
  {{/ifNth}}
  
  {{/each}}
</div> <!-- end row  -->
```