Feature nav plugin

### Overview

Nav plugin has specific syntax like haik-markdown.

```
::: nav right
* Item: http://www.example.com
* Dropdown
    * Sub Item: FooPage
    * ----
    * Sub Item: BarPage
:::
```

#### Tasks

- [x] Add NavParser
- [x] Enable right option
- [x] Enable form option
- [x] Enable divider in sub-list
- [x] Enable special attributtes
- [x] Add specific plugin repository providing button
- [x] Add specific Button plugin

#### Options

* right: add .navbar-right to ul element
* form: add .navbar-form to form element