Add urlFormatter option

This allows to specify a url formatter function. Example:

```javascript
var url = require('url');
var source = "[mylink](path/to/somewhere)";

marked.setOptions({
  urlFormatter: function(href) {
    return url.resolve('http://mydomain.com', href);
  }
});

console.log(marked(source)); // <p><a href="http://mydomain.com/path/to/somewhere">mylink</a></p>
```