Add icon of slack

Author: devjin0617

_config.yml

@@ -42,6 +42,7 @@ facebook_appid:
 facebook_comments_number: 10
 
 # Social icons
+slack_url: https://vuejs-korea.signup.team/
 github_username: vuejs-kr
 bitbucket_username:
 stackoverflow_id:

_includes/social_links.html

@@ -1,5 +1,8 @@
 <div class="social-icons">
   <div class="social-icons-right">
+    {% if site.slack_url %}
+      <a class="fa fa-slack" href="{{ site.slack_url }}"></a>
+    {% endif %}
     {% if site.github_username %}
       <a class="fa fa-github" href="https://github.com/{{ site.github_username }}"></a>
     {% endif %}