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/_data/authors.yml

@@ -2,17 +2,27 @@
 
 Billy Rick:
   name        : "Billy Rick"
-  uri         : "http://thewhip.com"
-  email       : "billy@rick.com"
   bio         : "What do you want, jewels? I am a very extravagant man."
   avatar      : "/assets/images/bio-photo-2.jpg"
-  twitter     : "extravagantman"
-  google_plus : "BillyRick"
+  links:
+    - label: "Email"
+      icon: "fas fa-fw fa-envelope-square"
+      url: "mailto:billyrick@rick.com"
+    - label: "Website"
+      icon: "fas fa-fw fa-link"
+      url: "https://thewhip.com"
+    - label: "Twitter"
+      icon: "fab fa-fw fa-twitter-square"
+      url: "https://twitter.com/extravagantman"
 
 Cornelius Fiddlebone:
   name        : "Cornelius Fiddlebone"
-  email       : "cornelius@thewhip.com"
   bio         : "I ordered what?"
   avatar      : "/assets/images/bio-photo.jpg"
-  twitter     : "rhymeswithsackit"
-  google_plus : "CorneliusFiddlebone"
+  links:
+    - label: "Email"
+      icon: "fas fa-fw fa-envelope-square"
+      url: "mailto:cornelius@thewhip.com"
+    - label: "Twitter"
+      icon: "fab fa-fw fa-twitter-square"
+      url: "https://twitter.com/rhymeswithsackit"