Add reference links support

Hi,
This commit tries to resolve #184.
I really want to have this feature so I decided to implement it by myself.

Known issue: the content in the `<meta name="description" content="">` tag would show "[text][id]" literally, instead of transforming it into "text".
```js
// more succinct
section.links = section.links || {};
// clearer
if (!section.links) section.links = {};
```