Add the canonical "footer" block

The base template and block system allows easy integration of themes with theme components.

The "footer" blocks is also one of the suggested blocks in the Hugo docs: https://gohugo.io/templates/base/#define-the-base-template.

Add this block quickly enables the use of theme components like [hugo-search-fuse-js](https://github.com/kaushalmodi/hugo-search-fuse-js/). I ended up creating this PR because of an issue created there: https://github.com/kaushalmodi/hugo-search-fuse-js/issues/1.

layouts/_default/baseof.html

@@ -8,6 +8,7 @@
     {{ block "header" . }}{{ partial "header.html" . }}{{ end }}
     {{ block "main" . }}{{ end }}
     {{ partial "footer.html" . }}
+    {{ block "footer" . }}{{ end }}
   </body>
 </html>