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.

It is available as `section.markup`.