feat(footer): add ability to auto update the copyright year

To use, add `{year}` to `Copyright` option in `config.toml`. Displays
current year.

Close #976

layouts/partials/footer_section.html

@@ -6,7 +6,7 @@
   {{ end }}
 
   <p class="powered-by">
-    {{ with .Site.Copyright }}{{ . | markdownify}} &middot; {{ end }}
+    {{ with .Site.Copyright }}{{ replace . "{year}" now.Year | markdownify}} &middot; {{ end }}
 
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