Add static grid gutters #8508

**Add static (unresponsive) grid gutters**
See: https://github.com/zurb/foundation-sites/issues/8508
Use: `. row.gutter-{size}`

Example:

```
<div class="row gutter-small"> <!-- Gutters has always "small" width -->
  <div class="large-6 columns">...</div>
  <div class="large-6 columns">...</div>
  ...
</div>
```

**Add grid-column-gutter mixin**
Set the gutters on a column.
Use: `@include grid-column-gutter($width);`