enable blog to redirect to asset (#550)

_posts/2022-02-01-redirect.md

@@ -0,0 +1,7 @@
+---
+layout: post
+title: a post with redirect
+date: 2021-07-04 17:39:00
+description: you can also redirect to assets like pdf
+redirect: /assets/pdf/example_pdf.pdf
+---

blog/index.html

@@ -30,7 +30,12 @@ pagination:
     {% assign categories = post.categories | join: "" %}
 
     <li>
-      <h3><a class="post-title" href="{{ post.url | prepend: site.baseurl }}">{{ post.title }}</a>
+      <h3>
+        {% if post.redirect == blank %}
+          <a class="post-title" href="{{ post.url | prepend: site.baseurl }}">{{ post.title }}</a>
+        {% else %}
+        <a class="post-title" href="{% if post.redirect contains '://' %}{{ post.redirect }}{% else %}{{ post.redirect | relative_url }}{% endif %}">{{ post.title }}</a>
+        {% endif %}
       </h3>
       <p>{{ post.description }}</p>
       <p class="post-meta"> {{read_time}} min read &nbsp; &middot; &nbsp;