Add an xxl grid tier

- Staring at my laptop, I realized I'm wasting away space. This adds a 
significant amount of CSS, but it feels right already.
- Added the xxl tier with a width that divides by 12
- Motivation was originally trying to have a third column for our toc so 
that our docs are more useful

scss/_variables.scss

@@ -276,7 +276,8 @@ $grid-breakpoints: (
   sm: 576px,
   md: 768px,
   lg: 992px,
-  xl: 1200px
+  xl: 1200px,
+  xxl: 1400px
 ) !default;
 
 @include _assert-ascending($grid-breakpoints, "$grid-breakpoints");
@@ -291,7 +292,8 @@ $container-max-widths: (
   sm: 540px,
   md: 720px,
   lg: 960px,
-  xl: 1140px
+  xl: 1140px,
+  xxl: 1320px
 ) !default;
 
 @include _assert-ascending($container-max-widths, "$container-max-widths");