Updates

This feature allows sub-tree directory parsing of `markdown` docs from `/docs` directory.

In few, now you can manage documents as:

```
/docs
|-- index.md
|-- format.md
|-- api
   |-- get.md
   |-- post.md
   |-- delete.md
...
```

Also add some docs on `node-markdocs` internal API with comment blocks.
:relaxed: