feat: Prevent vertical rhythm rules with variable

Fixes #147

fork-version/_normalize.scss

@@ -5,14 +5,16 @@
 @import 'vertical-rhythm';
 
 // If we've customized any font variables, we'll need extra properties.
-@if $base-line-height != 24px
-  or $base-unit != 'em'
-  or $h2-font-size != 1.5  * $base-font-size
-  or $h3-font-size != 1.17 * $base-font-size
-  or $h4-font-size != 1    * $base-font-size
-  or $h5-font-size != 0.83 * $base-font-size
-  or $h6-font-size != 0.67 * $base-font-size
-  or $indent-amount != 40px {
+@if $normalize-vertical-rhythm == null
+  and (
+    $base-line-height != 24px
+    or $base-unit != 'em'
+    or $h2-font-size != 1.5  * $base-font-size
+    or $h3-font-size != 1.17 * $base-font-size
+    or $h4-font-size != 1    * $base-font-size
+    or $h5-font-size != 0.83 * $base-font-size
+    or $h6-font-size != 0.67 * $base-font-size
+  ) {
   $normalize-vertical-rhythm: true !global;
 }
 

fork-version/_variables.scss

@@ -32,5 +32,6 @@ $indent-amount: 40px !default;
 // font-sizes, line-heights and block-level top/bottom margins that form a basic
 // vertical rhythm on the page, which differs from the original Normalize.css.
 // However, changing any of the variables above will cause
-// $normalize-vertical-rhythm to be automatically set to true.
-$normalize-vertical-rhythm: false !default;
+// $normalize-vertical-rhythm to be automatically set to true. To prevent
+// outputting any vertical rhythm rules, set this variable to false.
+$normalize-vertical-rhythm: null !default;

sass/normalize/_normalize-mixin.scss

@@ -34,13 +34,16 @@
   $init: _normalize-include($include, $exclude);
 
   // If we've customized any font variables, we'll need extra properties.
-  @if $base-line-height != 24px
-    or $base-unit != 'em'
-    or $h2-font-size != 1.5  * $base-font-size
-    or $h3-font-size != 1.17 * $base-font-size
-    or $h4-font-size != 1    * $base-font-size
-    or $h5-font-size != 0.83 * $base-font-size
-    or $h6-font-size != 0.67 * $base-font-size {
+  @if $normalize-vertical-rhythm == null
+    and (
+      $base-line-height != 24px
+      or $base-unit != 'em'
+      or $h2-font-size != 1.5  * $base-font-size
+      or $h3-font-size != 1.17 * $base-font-size
+      or $h4-font-size != 1    * $base-font-size
+      or $h5-font-size != 0.83 * $base-font-size
+      or $h6-font-size != 0.67 * $base-font-size
+    ) {
     $normalize-vertical-rhythm: true !global;
   }
 

sass/normalize/_variables.scss

@@ -32,5 +32,6 @@ $indent-amount: 40px !default;
 // font-sizes, line-heights and block-level top/bottom margins that form a basic
 // vertical rhythm on the page, which differs from the original Normalize.css.
 // However, changing any of the variables above will cause
-// $normalize-vertical-rhythm to be automatically set to true.
-$normalize-vertical-rhythm: false !default;
+// $normalize-vertical-rhythm to be automatically set to true. To prevent
+// outputting any vertical rhythm rules, set this variable to false.
+$normalize-vertical-rhythm: null !default;

test/fixtures/variables/prevent-vertical-rhythm/input.scss

@@ -0,0 +1,7 @@
+// This variable only affects output when vertical rhythm is used.
+$h2-font-size: 48px;
+// Force vertical rhythm off.
+$normalize-vertical-rhythm: false;
+
+@import 'normalize';
+@include normalize();