Add a facebook icon to Pixyll

_config.yml

# ...

# Social icons
show_social_icons: true
github_username:
stackoverflow_id:
twitter_username:
google_plus_id:
linkedin_username:
bitcoin_url:
paypal_url:
flattr_button:
facebook_id: 1234

# ...

_includes--header.html

<div class="social-icons">
  <div class="left">
    {% if site.github_username %}
      <a class="fa fa-github" href="https://github.com/{{ site.github_username }}"></a>
    {% endif %}
    {% if site.stackoverflow_id %}
      <a class="fa fa-stack-overflow" href="https://stackoverflow.com/users/{{ site.stackoverflow_id }}"></a>
    {% endif %}
    <a class="fa fa-rss" href="{{ "/feed.xml" | prepend: site.baseurl }}"></a>
    {% if site.twitter_username %}
      <a class="fa fa-twitter" href="https://twitter.com/{{ site.twitter_username }}"></a>
    {% endif %}
    {% if site.google_plus_id %}
      <a class="fa fa-google-plus" href="https://plus.google.com/{{ site.google_plus_id }}/posts"></a>
    {% endif %}
    {% if site.email %}
      <a class="fa fa-envelope" href="mailto:{{ site.email }}"></a>
    {% endif %}
    {% if site.linkedin_username %}
      <a class="fa fa-linkedin" href="https://www.linkedin.com/in/{{ site.linkedin_username }}"></a>
    {% endif %}
    {% if site.facebook_id %}
      <a class="fa fa-facebook-square" href="http://facebook.com/profile.php?id={{facebook_id}}"></a>
    {% endif %}
  </div>
  <div class="right">
    {% if site.bitcoin_url %}
      <a class="fa fa-bitcoin" href="{{ site.bitcoin_url }}"></a>
    {% endif %}
    {% if site.paypal_url %}
      <a class="fa fa-paypal" href="{{ site.paypal_url }}"></a>
    {% endif %}
    {% if site.flattr_button %}
      {{ site.flattr_button }}
    {% endif %}
  </div>
</div>
<div class="clearfix"></div>