Set width of `layout__item`'s to `auto` as default

I think it's useful to have the default `width` of `layout__item`'s to be `auto`. For example, if I want to have two elements next to each other, only taking up the necessary space, while being vertically aligned to the middle, I could do:

```html
<div class="layout layout--middle">
  <div class="layout__item"><h2>Some tall content</h2></div>
  <div class="layout__item">Some more content bumped up next to my sibling</div>
</div>
```

If I ever want to specify the `width` of a `layout__item`, I simply use the width classes, as mentioned in `objects.layout`'s docs:

```html
<div class="layout layout--middle">
  <div class="layout__item one-half"><h2>Some tall content</h2></div>
  <div class="layout__item one-half">Some content taking up the other half</div>
</div>
```

I'm not sure what the benefit is of the default `width` being `100%`. It seems like one could easily add the `one-whole` class to their `layout__item`'s to achieve it.