Featured articles

## THIS DEPENDS ON https://github.com/apiaryio/apiary/pull/1594

#### Description

Introducing featured articles for @neha3091. Usage:

```yaml
featured: true
```

...in meta part of the article.

Multiple articles can be featured, but it's not that nice as if only one is. If any, featured articles are listed at the top under "FEATURED POST" or "FEATURED POSTS" heading, chronologically. After them are listed all other articles, again, chronologically.
comment should be inside the `{if}`