feat: add custom social links

- add custom type for social.links. User can set type = custom with id
  as a custom URL.

  [[social.links]]
    type = "custom"
    id = "https://example.com/"
    icon = "fab fa-github"

layouts/partials/sidebar.html

@@ -66,6 +66,8 @@
         {{ $url = printf "mailto:%s" .id }}
       {{ else if eq .type "rss" }}
         {{ $url = "index.xml" | absLangURL }}
+      {{ else if eq .type "custom" }}
+        {{ $url = printf "%s" .id }}
       {{ else }}
         {{ $url = .url }}
       {{ end }}