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
The scenario I imagined for the bleed element was for things like carousels and hero units that might set their own margins on inner elements. Your example does make sense, although it seems a little counter-intuitive unless you know the implementation details (a sign of bad API design). I thought this might make more sense as a utility, but it actually makes it even less readable/grokkable.

Needs more consideration.