Add posts, categories, category, tags, tag, and collection layouts

Replace sample pages with hard coded HTML and Liquid in favor of a layout that does all the heavy lifting. Assign the appropriate `layout` via YAML Front Matter and away you go.

_layouts/tag.html

@@ -0,0 +1,9 @@
+---
+layout: archive
+---
+
+{{ content }}
+
+<div class="entries-{{ page.entries_layout | default: 'list' }}">
+  {% include posts-tag.html taxonomy=page.taxonomy %}
+</div>