[WIP] New extension method: "render"

You can use it in a sidebar of layout.html instead of `$toc` like this:

When:

```
// pages/nav.md
* [Aaa](aaa.html)
* [Bbb](bbb.html)
    - [Bbb part 1](bbb-1.html)
    - [Bbb part 2](bbb-2.html)
```

And:

```
// templates/layout.html
${render('nav.md')}
```

Then:

```
<ul>
    <li><a href="bbb-1.html">Bbb part 1</a></li>
    <li><a href="bbb-2.html">Bbb part 2</a></li>
</ul>
```