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/_pages/home.md

@@ -14,23 +14,26 @@ feature_row:
     title: "Super Customizable"
     excerpt: "Everything from the menus, sidebars, comments, and more can be configured or set with YAML Front Matter."
     url: "/docs/configuration/"
+    btn_class: "btn--primary"
     btn_label: "Learn More"
   - image_path: /assets/images/mm-responsive-feature.png
     alt: "fully responsive"
     title: "Responsive Layouts"
     excerpt: "Built on HTML5 + CSS3. All layouts are fully responsive with helpers to augment your content."
     url: "/docs/layouts/"
+    btn_class: "btn--primary"
     btn_label: "Learn More"
   - image_path: /assets/images/mm-free-feature.png
     alt: "100% free"
     title: "100% Free"
     excerpt: "Free to use however you want under the MIT License. Clone it, fork it, customize it, whatever!"
     url: "/docs/license/"
+    btn_class: "btn--primary"
     btn_label: "Learn More"
 github:
   - excerpt: '{::nomarkdown}<iframe style="display: inline-block;" src="https://ghbtns.com/github-btn.html?user=mmistakes&repo=minimal-mistakes&type=star&count=true&size=large" frameborder="0" scrolling="0" width="160px" height="30px"></iframe> <iframe style="display: inline-block;" src="https://ghbtns.com/github-btn.html?user=mmistakes&repo=minimal-mistakes&type=fork&count=true&size=large" frameborder="0" scrolling="0" width="158px" height="30px"></iframe>{:/nomarkdown}'
 intro:
-  - excerpt: 'Get notified when I add new stuff &nbsp; [<i class="fa fa-twitter"></i> @mmistakes](https://twitter.com/mmistakes){: .btn .btn--twitter} [<i class="fa fa-paypal"></i> Tip Me](https://www.paypal.me/mmistakes){: .btn}'
+  - excerpt: 'Get notified when I add new stuff &nbsp; [<i class="fa fa-twitter"></i> @mmistakes](https://twitter.com/mmistakes){: .btn .btn--twitter} [<i class="fa fa-paypal"></i> Tip Me](https://www.paypal.me/mmistakes){: .btn .btn--primary}'
 ---
 
 {% include feature_row id="intro" type="center" %}