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.

_includes/posts-category.html

@@ -0,0 +1,3 @@
+{%- for post in site.categories[include.taxonomy] -%}
+  {% include archive-single.html %}
+{%- endfor -%}