Allow customized copyright contents (#254)

Author: shenchsh

layout/_macro/post-copyright.swig

@@ -9,6 +9,6 @@
   </li>
   <li class="post-copyright-license">
     <strong>{{ __('post.copyright.license_title') + __('symbol.colon') }} </strong>{#
-  #}{{ __('post.copyright.license_content', theme.post_copyright.license) }}{#
+  #}{{ post.copyright | default(__('post.copyright.license_content', theme.post_copyright.license)) }}{#
 #}</li>
 </ul>