Rows without gutters

Hi! 
What about a `l-row--no-gutter` class, even if that requires a level of nesting?

```
  .l-row
    -  .l-col--xx-xx
    -  [...]
  .l-row--no-gutter
    -  .l-col--xx-xx
    -  [...]
```

This way all the columns code would be duplicated, but that shouldn't be an issue.
What do you think about it? Could it be useful?

I could make a PR for this, too!