Add Twitter card. relates to #16

_includes/head.html

@@ -13,4 +13,6 @@
   {% if jekyll.environment == 'production' and site.google_analytics %}
   {% include google-analytics.html %}
   {% endif %}
+
+  {% include twitter-card.html %}
 </head>

_includes/twitter-card.html

@@ -0,0 +1,10 @@
+<!-- Twitter Card -->
+<meta name="twitter:card" content="{% if page.image %}summary_large_image{% else %}summary{% endif %}">
+<meta name="twitter:site" content="@{{ site.twitter_username }}">
+<meta name="twitter:creator" content="@{{ site.twitter_username }}">
+<meta name="twitter:title" content="{% if page.title %}{{ page.title }}{% else %}{{ site.title }}{% endif %}">
+<meta name="twitter:description" content="{% if page.excerpt %}{{ page.excerpt | strip_html | strip_newlines | truncate: 200 }}{% else %}{{ site.description}}{% endif %}">
+{% if page.image %}
+    <meta name="twitter:image:src" content="{{ page.image | prepend: site.baseurl | prepend: site.url }}">
+{% endif %}
+<!-- End Twitter Card -->