Added support for align-items: stretch in grid

Now you can do:

```
<div class="row row-stretch">
    <div class="col">Very<br />Much<br />Text<br /></div>
    <div class="col">Short</div>
    <div class="col">Shrt</div>
</div>
```

All columns will have equal heights. Especially useful when 'simulating' tables with the grid system.