Add Content component

Add a component for consistent horizontal margins on content. Also allows making descendant containers “full-bleed” by negating those  margins.

Status: **Opened for visibility**

Reviewers: **@jeffkamo @kpeatt **

## Changes
- Add content component
- NOTE: this uses the namespaced variables proposed in #59 

## Todo
- [ ] Add tests
For this sub-component, should it have a padding?

Left by itself, the `.c-content__bleed` element will space full width, but so will the content inside it.

Are you perhaps expecting something like this?

```
<div class="c-content">
    <div class="c-content__bleed c-content></div>
</div>
```

...such that the bleed element's content can have padding and align with the parent's content?