adding mastodon social link (#24)

Author: jchiquet

_config.yml

@@ -65,7 +65,7 @@ og_image: # The site-wide (default for all links) Open Graph preview image
 
 github_username: computorg
 twitter_username: # your Twitter handle
-mastodon_username: # your mastodon instance+username in the format instance.tld/@username
+mastodon_username: mathstodon.xyz/@computo # your mastodon instance+username in the format instance.tld/@username
 linkedin_username: # your LinkedIn user name
 telegram_username: # your Telegram user name
 scholar_userid: # your Google Scholar ID

_includes/header.html

@@ -81,10 +81,12 @@
             </li>
             {% endif %}
 
-	  	
-            <li>&nbsp; &nbsp;
+            <li>&nbsp;
               <a class="navbar-brand title font-weight-lighter" href="https://github.com/{{ site.github_username }}" target="_blank" title="GitHub"><i class="fab fa-github"></i></a>
             </li>
+            <li>
+              <a class="navbar-brand title font-weight-lighter" href="https://{{ site.mastodon_username }}" target="_blank" title="Mastodon"><i class="fab fa-mastodon"></i></a>
+            </li>
         </ul>
 
       </div>