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
That's quite interesting. I didn't know you could do that.

Perhaps having a real life example would help clarify this paragraph?