Added layout for multiple categories

Signed-off-by: Jim Martens <github@2martens.de>

_layouts/categories.html

@@ -0,0 +1,23 @@
+---
+layout: default
+---
+
+{{ content }}
+<ul class="post-list list-unstyled">
+    {% assign categories = page.categories | split:',' %}
+    {% assign posts = site.categories[categories.first] %}
+    {% for category in categories %}
+        {% if category != categories.first %}
+            {% assign posts = site.categories[category] | concat: posts %}
+        {% endif %}
+    {% endfor %}
+    {% assign posts = posts | sort: 'date' | reverse %}
+    {% for post in posts %}
+    <li class="mb-4">
+        <span class="small text-muted">{{ post.date | date: "%b %-d, %Y" }}</span>
+        <p class="h4 post-title">
+            <a class="post-link" href="{{ post.url | absolute_url }}">{{ post.title }}</a>
+        </p>
+    </li>
+    {% endfor %}
+</ul>