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/skins/_dirt.scss

@@ -0,0 +1,15 @@
+/* ==========================================================================
+   Dirt skin
+   ========================================================================== */
+
+/* Colors */
+$background-color: #f3f3f3 !default;
+$text-color: #343434 !default;
+$muted-text-color: #8e8b82 !default;
+$primary-color: #343434 !default;
+$border-color: #e9dcbe !default;
+$footer-background-color: #e9dcbe !default;
+$link-color: #343434 !default;
+$masthead-link-color: $text-color !default;
+$masthead-link-color-hover: $text-color !default;
+$navicon-link-color-hover: mix(#fff, $text-color, 80%) !default;