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/_page.scss

@@ -91,8 +91,7 @@
     }
   }
 
-  a {
-    text-decoration: none;
+  a:not(.btn) {
 
     &:hover {
       text-decoration: underline;
@@ -250,7 +249,7 @@
 
 .page__meta {
   margin-top: 2em;
-  color: mix(#fff, $gray, 25%);
+  color: $muted-text-color;
   font-family: $sans-serif;
   font-size: $type-size-6;
 
@@ -290,7 +289,7 @@
   margin-bottom: 8px;
   padding: 5px 10px;
   text-decoration: none;
-  border: 1px solid $light-gray;
+  border: 1px solid mix(#000, $border-color, 25%);
   border-radius: $border-radius;
 
   &:hover {
@@ -322,8 +321,6 @@
 }
 
 .page__comments-form {
-  padding: 1em;
-  background: $lighter-gray;
   -webkit-transition: $global-transition;
   transition: $global-transition;