Improve author link flexibility

Deprecate social networking links in `_config.yml` in favor or new `author.links` array for any link and in any order. All of Font Awesome's icons are available for use.

```
author:
  links:
    - label: "Your Website"
      icon: "fas fa-fw fa-link"
      url: "https://your-site.com"
    - label: "Twitter"
      icon: "fab fa-fw fa-twitter-square"
      url: "https://twitter.com/username"
    - label: "GitHub"
      icon: "fab fa-fw fa-github"
      url: "https://github.com/username"
    - label: "Instagram"
      icon: "fab fa-fw fa-instagram"
      url: "https://instagram.com/username"
```

Fixes #1581

_includes/author-profile.html

@@ -48,6 +48,14 @@
         </li>
       {% endif %}
 
+      {% if author.links %}
+        {% for link in author.links %}
+          {% if link.label and link.url %}
+            <li><a href="{{ link.url }}"><i class="{{ link.icon | default: 'fas fa-link' }}" aria-hidden="true"></i> {{ link.label }}</a></li>
+          {% endif %}
+        {% endfor %}
+      {% endif %}
+
       {% if author.uri %}
         <li>
           <a href="{{ author.uri }}" itemprop="url">