set default width to 100%

Instead of doing this
```HTML
<div class="grid">
    <div class="grid__col grid__col--s12">...</div>
    <div class="grid__col grid__col--s12">...</div>
</div>
```
now we can do this
```HTML
<div class="grid">
    <div class="grid__col">...</div>
    <div class="grid__col">...</div>
</div>
```