Add support for OpenGraph meta tags in layouts

OpenGraph Protocol tags are now generated in the `<head>` element of each page. Example:

```html
        <meta name="fb:description" content="November 2014 Meetup Schedule">
        <meta name="fb:image" content="http://paperswelove.org/images/papers-we-love-ogp.png">
        <meta name="fb:image:type" content="image/png">
        <meta name="fb:image:width" content="400">
        <meta name="fb:image:height" content="300">
        <meta name="og:description" content="November 2014 Meetup Schedule">
        <meta name="og:image" content="http://paperswelove.org/images/papers-we-love-ogp.png">
        <meta name="og:image:type" content="image/png">
        <meta name="og:image:width" content="400">
        <meta name="og:image:height" content="300">
        <meta name="og:locale" content="en_us">
        <meta name="og:type" content="article">
        <meta name="og:title" content="November Meetups">
        <meta name="og:url" content="http://paperswelove.org/2014/news/november-meetups/">
        <meta name="article:published_time" content="2014-11-01T00:00:00Z">
        <meta name="article:tag" content="meetup">
        <meta name="article:tag" content="chapters">
```