Implement displaying individual components on the style guide

## Implement displaying individual components on the style guide

### Acceptance Criteria
1. Clicking on a component name in the nav updates the page to show just that component.
1. Clicking on "Kitchen Sink" goes back to showing everything.

### Tasks
- Create StyleGuideNavItem component that returns a style guide section's display name on click.
- Listen for on click in StyleGuideHeader and add the display name to the route.
- Add conditional logic to the StyleGuide component's render method so that it only displays the section whose display name matches the one in the route (or display everything if there isn't one).

### Additional Notes
- None