Fallback to github image if gravatar is nil (#479)

Updates the post layout to fallback to the author's github profile image
if their gravatar is not set.

Fixes #476

Author: rauhul

_layouts/post.html

@@ -13,6 +13,8 @@ <h1>{{ page.title }}</h1>
       <div class="byline">
         {% if author.gravatar %}
           <img src="https://www.gravatar.com/avatar/{{ author.gravatar }}?s=64&d=mp" alt="{{ author.name }}"/>
+        {% elsif author.github %}
+          <img src="https://www.github.com/{{ author.github }}.png?size=64" alt="{{ author.name }}"/>
         {% else %}
           <img src="https://www.gravatar.com/avatar/dummy?s=64&d=mp&f=y" alt="{{ author.name }}"/>
         {% endif %}