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

tests/plugins/PluginMarkdownTest.php

@@ -47,6 +47,8 @@ class PluginMarkdownTest extends PHPUnit_Framework_TestCase
         $data = hook_markdown_render_linklist($data, $this->conf);
         $this->assertNotFalse(strpos($data['links'][0]['description'], '<h1>'));
         $this->assertNotFalse(strpos($data['links'][0]['description'], '<p>'));
+
+        $this->assertEquals($markdown, $data['links'][0]['description_src']);
     }
 
     /**