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

docs/_sass/minimal-mistakes/_sidebar.scss

@@ -189,7 +189,7 @@
   list-style-type: none;
   border: 1px solid $border-color;
   border-radius: $border-radius;
-  background: #fff;
+  background: $background-color;
   z-index: -1;
   box-shadow: 0 2px 4px 0 rgba(#000, 0.16), 0 2px 10px 0 rgba(#000, 0.12);
   cursor: default;
@@ -234,7 +234,7 @@
     width: 0;
     border-style: solid;
     border-width: 0 10px 10px;
-    border-color: #fff transparent;
+    border-color: $background-color transparent;
     z-index: 1;
 
     @include breakpoint($large) {