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
No, it’s correct, but maybe confusing? Basically the Content element maxes out at the specified width, so when the container exceeds that width the Content element becomes horizontally centered within it.