Refactor tabs implementation (#3268)

* Refactor tabs implementation

* tidy head.html

* test jquery-ui tabs

* Initial tabs code refactor

* test liquid parsing

* test liquid stack

* move into tabs.html

* test jekyll tabs

* construct array

* test cleaner way

* add example content

* test split hack

* fix md

* add site.emptyArray

* set good defaults

* fix CSS

* cleanup li

* fix a href

* fix content tabs

* add space to default

* remove a href underline

* rename docs/tab-test.md to docs/tab-example.md

* fix skip toc check

* add assignees

* add comment to reference example

* add documentation to example

* escape md

* move demo to top, pretty it up

* add word

* tweak wording

_includes/tabs.md

@@ -0,0 +1,15 @@
+{% comment %} See /docs/tabs-example.md for usage example. {% endcomment %}
+{% assign tab_set_id = tab_set_name | default: "tabset" | slugify %}
+<div id="{{tab_set_id}}">
+    <ul>
+{% for name in tab_names %}
+        <li><a href="#{{tab_set_id}}-{{forloop.index0}}">{{ name | strip }}</a></li>
+{% endfor %}
+    </ul>
+{% for content in tab_contents %}
+    <div id="{{tab_set_id}}-{{forloop.index0}}">
+    {{ content | markdownify }}
+    </div>
+{% endfor %}
+</div>
+<script>$(function(){$("#{{tab_set_id}}").tabs();});</script>