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/_syntax.scss

@@ -19,7 +19,7 @@ figure.highlight {
     top: 0;
     right: 0;
     padding: 0.5em;
-    background-color: $lighter-gray;
+    background-color: $border-color;
     content: "\f121";
     font-family: "fontawesome" !important;
     line-height: 1;
@@ -49,7 +49,7 @@ figure.highlight {
     /* line numbers*/
     &.gutter {
       padding-right: 1em;
-      color: $light-gray;
+      color: rgba($muted-text-color, 0.5);
     }
   }