Make the component full-width by default

This makes it easier for the component to work in any parent context. Would require a major version release as it would break components relying on the default `Button`'s current rendering (inline-block by default). 

The parent context can define the spacing around the button, whether it's block or inline-block, etc.

For example:

```html
<!-- Inline block -->
<div class="u-inlineBlock">
  <button class="Button">Button</button>
</div>

<!-- Left margin, inline-block -->
<div class="LoginForm-submit">
  <button class="Button">Button</button>
</div>
```