Add feature: use api.posts.featured(opts) to list all featured posts.

A new feature in feature list, 
now can use 
```
api.posts.featured(opts) 
```
to list all featured posts in controller

opts.limit set the count of returns

In my code we can use like

```
 {{#foreach featuredPosts}}
 {{{title}}}
 {{/foreach}}
```
 
to show then~