Add keybase badge (alshedivat#132)

Author: shpark

_config.yml

@@ -49,6 +49,7 @@ quora_username: # your Quora username
 publons_id: # your ID on Publons
 research_gate_profile: # your profile on ResearchGate
 blogger_url: # your blogger URL
+keybase_username: # your keybase user name
 
 contact_note: >
   You can even add a little note about which of these is the best way to reach you.

_includes/social.html

@@ -11,4 +11,5 @@
   {% if site.quora_username %}<a href="https://www.quora.com/profile/{{ site.quora_username }}" target="_blank" title="Quora"><i class="fab fa-quora"></i></a>{% endif %}
   {% if site.blogger_url %}<a href="{{ site.blogger_url }}" target="_blank" title="Blogger"><i class="fab fa-blogger-b"></i></a>{% endif %}
   {% if site.strava_userid %}<a href="https://www.strava.com/athletes/{{ site.strava_userid }}" target="_blank" title="Strava"><i class="fab fa-strava"></i></a>{% endif %}
+  {% if site.keybase_username %}<a href="https://keybase.io/{{ site.keybase_username }}" target="_blank" title="Keybase"><i class="fab fa-keybase"></i></a>{% endif %}
 </span>