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/"
 
 
 # Reading Files

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"

test/_posts/2012-03-15-layout-author-override.md

@@ -8,25 +8,37 @@ Sites that may have content authored from various individuals can be accommodate
 
 To attribute an author to a post or page that is different from the site author specified in `_config.yml`:
 
-**Step 1.** Create `_data/authors.yml` and add authors using the following format. Anything variables found under `author` in `_config.yml` can be used (e.g. `name`, `avatar`, `uri`, social media profiles, etc.).
+**Step 1.** Create `_data/authors.yml` and add authors using the following format. Anything variables found under `author` in `_config.yml` can be used (e.g. `name`, `bio`, `avatar`, author `links`, etc.).
 
 ```yaml
 # /_data/authors.yml
 
 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"
+  name        : "Billy Rick"
+  bio         : "What do you want, jewels? I am a very extravagant man."
+  avatar      : "/assets/images/bio-photo-2.jpg"
+  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"
+  name        : "Cornelius Fiddlebone"
+  bio         : "I ordered what?"
+  avatar      : "/assets/images/bio-photo.jpg"
+  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"
 ```
 
 **Step 2.** Assign one of the authors in `authors.yml` to a post or page you wish to override the `site.author` with.