Fixes #17: Feature a post

From #17:

One should be able to make a post a "featured post". There should only be one of these. The selection for the featured post shall be on the list of posts page.

If there is a featured post selected, it will be included in a new variable to be rendered, separate from the $miniblog_posts variable. An implementing developer will be able to echo the $featured_post into some place as desired, and it will follow the defined post template.

@gholtslander @jrans-va

I have included sample usage in the `index.php` file as well, showing that it will only appear if there is a featured post, and if they are not viewing a single post. In some cases, some people may want their featured post shown all the time, regardless of where they're looking (Dad, I think this would be appropriate in the case of the turfit site).
In this example, the featured post is shown in a Bootstrap "Jumbotron" element, but only if there is a featured post in the first place, and as long as they are not viewing a single post.