Added support for reference hooks

Allows the renderer to automatically resolve references if they are not defined in the document. This is useful for wiki-type sites.

### Example
```javascript
renderer.reference = function (name) {
   return '/articles/' + name.toLowerCase().replace(' ', '-');
}
```
### Example Input
```markdown
Node.js is powered by [v8]
```

### Example Output
```markdown
<p>Node.js is powered by <a href="/articles/v8">v8</a></p>
```