add stackoverflow id (alshedivat#400)

Author: pourmand1376

_config.yml

@@ -64,6 +64,7 @@ work_url: # work page URL
 keybase_username: # your keybase user name
 wikidata_id: # your wikidata id
 dblp_url: # your DBLP profile url
+stackoverflow_id: #your stackoverflow id
 
 contact_note: >
   You can even add a little note about which of these is the best way to reach you.

_includes/social.html

@@ -15,3 +15,5 @@
 {% if site.keybase_username %}<a href="https://keybase.io/{{ site.keybase_username }}" target="_blank" title="Keybase"><i class="fab fa-keybase"></i></a>{% endif %}
 {% if site.gitlab_username %}<a href="https://gitlab.com/{{ site.gitlab_username }}" target="_blank" title="GitLab"><i class="fab fa-gitlab"></i></a>{% endif %}
 {% if site.dblp_url %}<a href="{{ site.dblp_url }}" target="_blank" title="DBLP"><i class="ai ai-dblp"></i></a>{% endif %}
+{% if site.stackoverflow_id %}<a href="https://stackoverflow.com/users/{{ site.stackoverflow_id }}" target="_blank" title="Stackoverflow"><i class="fab fa-stack-overflow"></i></a>{% endif %}
+