make social links optional

Author: Steve Ortiz

_includes/social.html

@@ -1,8 +1,16 @@
 <div class="navbar social">
   <ul class="nav navbar-nav">
-    <li><a href="https://www.linkedin.com/in/{{ site.linkedin_username }}"><i class="fa fa-linkedin-square"></i></a></li>
-    <li><a href="https://github.com/{{ site.github_username }}"><i class="fa fa-github"></i></a></li>
-    <li><a href="https://twitter.com/{{ site.twitter_username }}"><i class="fa fa-twitter"></i></a></li>
-    <li><a href="mailto:{{ site.email }}"><i class="fa fa-envelope"></i></a></li>
+    {% if site.linkedin_username %}
+      <li><a href="https://www.linkedin.com/in/{{ site.linkedin_username }}"><i class="fa fa-linkedin-square"></i></a></li>
+    {% endif %}
+    {% if site.github_username %}
+      <li><a href="https://github.com/{{ site.github_username }}"><i class="fa fa-github"></i></a></li>
+    {% endif %}
+    {% if site.twitter_username %}
+      <li><a href="https://twitter.com/{{ site.twitter_username }}"><i class="fa fa-twitter"></i></a></li>
+    {% endif %}
+    {% if site.email %}
+      <li><a href="mailto:{{ site.email }}"><i class="fa fa-envelope"></i></a></li>
+    {% endif %}
   </ul>
 </div>