footer: implement user-defined copyright (#23)

layouts/partials/footer.html

@@ -1,5 +1,9 @@
 <footer class="footer">
+    {{- if .Site.Copyright }}
+    <span>{{ .Site.Copyright | markdownify }}</span>
+    {{- else }}
     <span>&copy; {{ now.Year }} <a href="{{ "" | absLangURL }}">{{ .Site.Title }}</a></span>
+    {{- end }}
     <span>&middot;</span>
     <span>Powered by <a href="https://gohugo.io/" rel="noopener noreferrer" target="_blank">Hugo️️</a>️</span>
     <span>&middot;</span>
@@ -62,4 +66,4 @@
 <script>
     localStorage.removeItem("pref-theme");
 </script>
-{{- end }}
+{{- end }}