Improve footer link flexibility

Deprecate hardcoded footer links in favor or new `footer.links` array for any link and in any order. All of Font Awesome's icons are available for use.

```
footer:
  links:
    - 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"
```

_config.yml

@@ -125,6 +125,28 @@ author:
       icon: "fab fa-fw fa-instagram"
       # url: "https://instagram.com/"
 
+# Site Footer
+footer:
+  links:
+    - label: "Twitter"
+      icon: "fab fa-fw fa-twitter-square"
+      # url:
+    - label: "Facebook"
+      icon: "fab fa-fw fa-facebook-square"
+      # url:
+    - label: "GitHub"
+      icon: "fab fa-fw fa-github"
+      # url:
+    - label: "GitLab"
+      icon: "fab fa-fw fa-gitlab"
+      # url:
+    - label: "Bitbucket"
+      icon: "fab fa-fw fa-bitbucket"
+      # url:
+    - label: "Instagram"
+      icon: "fab fa-fw fa-instagram"
+      # url:
+
 
 # Reading Files
 include: