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
- [ ] Enable right option
- [ ] Enable form option
- [ ] Enable separator in sub-list
- [ ] Add specific plugin repository providing button
- [ ] Add specific Button plugin

#### Options

* right: add .navbar-right to ul element
* form: add .navbar-form to form element