Add class 'styleguide' to hologram elements

This PR adds a class 'styleguide' to elements generated from markdown documentation. We expect this will allow a cleaner separation between styleguide specific styles and actual site styles. We're submitting this because we had trouble styling the styleguide's tables without affecting the style of our table components.

We expect it to be used as follows:

```css
table.styleguide { border: 1px solid gray; }
h1.styleguide { color: teal; }
```

Feedback?

Nicole & @gpleiss