Add [Fraction:x/y] Markdown helper

This is automatically converted into a SUB/SUP construct with the correct HTML entity for a fraction divider between it.

Example:

```markdown
[Fraction:1/2]
```

becomes:

```html
<span class="fraction"><sup>1</sup>&frasl;<sub>2</sub></span>
```

`&frasl;` is the HTML entity for fraction separator.

This is the first part of https://www.pivotaltracker.com/story/show/56342014 .