facebook sharing image meta tag added related #149 issue

Author: kishore-mohan

_includes/_open-graph.html

@@ -16,4 +16,9 @@
 <meta property="og:title" content="{% if page.title %}{{ page.title }}{% else %}{{ site.title }}{% endif %}">
 {% if page.excerpt %}<meta property="og:description" content="{{ page.excerpt | strip_html }}">{% endif %}
 <meta property="og:url" content="{{ page.url | replace:'index.html','' | prepend: site.url }}">
-<meta property="og:site_name" content="{{ site.title }}">
+<meta property="og:site_name" content="{{ site.title }}">
+{% if page.image.feature %}
+<meta property="og:image" content="{{ site.url }}/images/{{ page.image.feature }}">
+{% else %}
+<meta property="og:image" content="{% if page.image.thumb %}{{ site.url }}/images/{{ page.image.thumb }}{% else %}{{ site.url }}/images/default-thumb.png{% endif %}">
+{% endif %}