Fixed figure margin (normalizing)

webkit applies some default margins to the `figure` element:
```css
-webkit-margin-before: 1em;
-webkit-margin-after: 1em;
-webkit-margin-start: 40px;
-webkit-margin-end: 40px;
```

In order to use the library out of the box without any (not yet mentioned) third-party libraries the `figure` element has to be normalized by setting the margin to 0.