Parse a markup section

Parses out a `Markup:` section like [kss-node](https://github.com/hughsk/kss-node#differences).

```scss
// Your standard form button.
// 
// Markup:
// <span class="button {$modifiers}">Example</span>
// <span class="other-stuff">Other stuff</span>
// 
// :hover    - Highlights when hovering.
// :disabled - Dims the button when disabled.
// .primary  - Indicates button is the primary action.
// .smaller  - A smaller button
// 
// Styleguide 2.1.1.
```

This allows the markup to be near the documentation. Note that it does not require a markup block to be there, but will parse it out if there is one. 

It is available as `section.markup`.