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

_sass/minimal-mistakes/_archive.scss

@@ -26,7 +26,7 @@
   margin: 1.414em 0 0;
   padding-bottom: 0.5em;
   font-size: $type-size-5;
-  color: mix(#fff, $gray, 25%);
+  color: $muted-text-color;
   border-bottom: 1px solid $border-color;
 
   + .list__item .archive__item-title {
@@ -72,6 +72,7 @@
 }
 
 .archive__item:hover {
+
   .archive__item-teaser {
     box-shadow: 0 0 10px rgba(#000, 0.25);
   }