shortcodes: Add list_tags and list_categories

To use:

`{{< list_tags >}}` to provide a list of linked tags.
`{{< list_categories >}}` to provide a list of linked categories.

layouts/shortcodes/list_categories.html

@@ -0,0 +1,5 @@
+<ul class="list-unstyled">
+  {{ range ($.Site.GetPage "taxonomyTerm" "categories").Pages }}
+  <li><a href="{{.Permalink}}">{{.Title}}</a></li>
+  {{ end }}
+</ul>

layouts/shortcodes/list_tags.html

@@ -0,0 +1,5 @@
+<ul class="list-unstyled">
+  {{ range ($.Site.GetPage "taxonomyTerm" "tags").Pages }}
+  <li><a href="{{.Permalink}}">{{.Title}}</a></li>
+  {{ end }}
+</ul>