Add OpenGraph meta tags on permalink page

Includes:
  - og:title
  - og:type -> article
  - og:image -> if there is a thumbnail
  - og:url -> permalink
  - og:description -> first 300 chars of raw description
  - article:published_time
  - article:modified_time
  - article:tag -> one OG meta tag for each shaare tag

Fixes #258

plugins/markdown/markdown.php

@@ -28,6 +28,7 @@ function hook_markdown_render_linklist($data, $conf)
             $value = stripNoMarkdownTag($value);
             continue;
         }
+        $value['description_src'] = $value['description'];
         $value['description'] = process_markdown(
             $value['description'],
             $conf->get('security.markdown_escape', true),