Make predefined grid classes conditional

- Add explicit -grid-classes variable and set to true by default
- Wrap .row and .col generation in the boolean var

scss/_grid.scss

@@ -28,8 +28,10 @@
 //
 // Rows contain and clear the floats of your columns.
 
-.row {
-  @include make-row();
+@if $enable-grid-classes {
+  .row {
+    @include make-row();
+  }
 }
 
 
@@ -37,14 +39,16 @@
 //
 // Common styles for small and large grid columns
 
-@include make-grid-columns();
+@if $enable-grid-classes {
+  @include make-grid-columns();
+}
 
 
 // Flex variation
 //
 // Custom styles for additional flex alignment options.
 
-@if $enable-flex {
+@if $enable-flex and $enable-grid-classes {
 
   // Flex column reordering
 

scss/_variables.scss

@@ -49,6 +49,7 @@ $enable-shadows:            false !default;
 $enable-gradients:          false !default;
 $enable-transitions:        false !default;
 $enable-hover-media-query:  false !default;
+$enable-grid-classes:       true !default;
 
 
 // Spacing