Supports links to Mastodon

Adds a link to Mastodon in the footer. This can be configured in the
same way as the existing icons for github, twitter, etc...

_config.yml

@@ -34,7 +34,7 @@ footer-links:
   youtube: # channel/<your_long_string> or user/<user-name>
   googleplus: # anything in your profile username that comes after plus.google.com/
   playconsole:
-
+  mastodon: # <your-mastodon-instance>/@<your-user-name> eg. fosstodon.org/@username
 
 # Enter your Disqus shortname (not your username) to enable commenting on posts
 # You can find your shortname on the Settings page of your Disqus account

_includes/svg-icons.html

@@ -12,3 +12,4 @@
 {% if site.footer-links.youtube %}<a href="https://youtube.com/{{ site.footer-links.youtube }}"><i class="svg-icon youtube"></i></a>{% endif %}
 {% if site.footer-links.googleplus %}<a href="https://plus.google.com/{{ site.footer-links.googleplus }}"><i class="svg-icon googleplus"></i></a>{% endif %}
 {% if site.footer-links.playconsole %}<a href="https://play.google.com/store/apps/dev?id={{ site.footer-links.playconsole }}"><i class="svg-icon playconsole"></i></a>{% endif %}
+{% if site.footer-links.mastodon %}<a rel="me" href="https://{{ site.footer-links.mastodon }}"><i class="svg-icon mastodon"></i></a>{% endif %}