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

test/_config.yml

@@ -91,31 +91,19 @@ author:
   avatar           : "/assets/images/bio-photo.jpg"
   bio              : "I am an amazing person."
   location         : "Somewhere"
-  email            :
-  uri              :
-  home             : # null (default), "absolute or relative url to link to author home"
-  bitbucket        :
-  codepen          :
-  dribbble         :
-  flickr           :
-  facebook         :
-  foursquare       :
-  github           :
-  google_plus      :
-  keybase          :
-  instagram        :
-  lastfm           :
-  linkedin         :
-  pinterest        :
-  soundcloud       :
-  stackoverflow    : # "123456/username" (the last part of your profile url, e.g. http://stackoverflow.com/users/123456/username)
-  steam            :
-  tumblr           :
-  twitter          : "mmistakes"
-  vine             :
-  weibo            :
-  xing             :
-  youtube          : # "https://youtube.com/c/MichaelRoseDesign"
+  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/"
+    - label: "GitHub"
+      icon: "fab fa-fw fa-github"
+      url: "https://github.com/"
+    - label: "Instagram"
+      icon: "fab fa-fw fa-instagram"
+      url: "https://instagram.com/"
 
 
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