Color "skins" (#1242)

* Add YIQ Color Contrast mixin
* Remove text underline from `.btn` links
* Move YIQ variables
* Simplify button classes using YIQ color contrast mixin
  - DRY up button CSS using Sass list and YIQ color contrast mixin.
  - Move `color` and `background-color` to new `btn--primary` class instead of assinging on the default class. Removes the need to override them.
* Add `.btn--primary` to buttons that just had `.btn`
* Apply changes to `/docs`
* Add `.btn--primary` class
* Update CHANGELOG and history
* Add sample form
* Abstract colors away into Sass variables for easier themeing
* Add "dark" skin
* Replace hardcoded color with SCSS variable
* Invert Font Awesome icons' colors in author sidebar and footer
* Add Sass changes to `/docs`
* Use primary button type instead of inverse
* Add missing `!default` on `$muted-text-color`
* Add `contrast` and `sunrise` skin colors
* Add `dirt` skin color
* Add `air` skin color option
* Add `mint` skin color
* Add `btn--primary` class to Submit Comment button
* Set skin to `default`
* Document skin color options
* Add note about skin SCSS import

Close #1208

_layouts/single.html

@@ -35,7 +35,7 @@ layout: default
 
       <section class="page__content" itemprop="text">
         {{ content }}
-        {% if page.link %}<div><a href="{{ page.link }}" class="btn">{{ site.data.ui-text[site.locale].ext_link_label | default: "Direct Link" }}</a></div>{% endif %}
+        {% if page.link %}<div><a href="{{ page.link }}" class="btn btn--primary">{{ site.data.ui-text[site.locale].ext_link_label | default: "Direct Link" }}</a></div>{% endif %}
       </section>
 
       <footer class="page__meta">