Allow customizing post date format

Now you can set the format of the date used in the post in the config:

```toml
[extra]
post_date_format = "%Y-%m-%d"
```

templates/macros/post.html

@@ -26,7 +26,7 @@
 {% macro date(page) %}
     <span class="post-date">
         {%- if page.date %}
-            {{ page.date | date(format="%Y-%m-%d") }}
+            {{ page.date | date(format=config.extra.post_date_format) }}
         {% endif -%}
     </span>
 {% endmacro post_date %}