Allowing to change the nth-selector (nth-child or nth-of-type) for the mixin isolate-grid

I find that it is interesting to be able to change the nth-selector in the *isolate-grid* mixin.
For instance in the following example:

```html
<section class="wrapper">
    <header><h2>section header</h2></header>
    <section class="someclass">... some content ...</section>
    <section class="someclass">... some content ...</section>
    <section class="someclass">... some content ...</section>
</section>
```

```css
section.wrapper section.someclass {
    @include isolate-grid(x/3, x, $nth-selector: 'of-type');
}
```

Therefore I just added the parameter *$nth-selector* in the mixin.

Best,
Rémi