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"
```

test/_config.yml

@@ -106,6 +106,20 @@ author:
       url: "https://instagram.com/"
 
 
+# Site Footer
+footer:
+  links:
+    - 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
 include:
   - .htaccess