Allow to customize vertical margin of `nav-divider` mixin

scss/mixins/_nav-divider.scss

@@ -2,9 +2,9 @@
 //
 // Dividers (basically an hr) within dropdowns and nav lists
 
-@mixin nav-divider($color: #e5e5e5) {
+@mixin nav-divider($color: #e5e5e5, $margin-y: ($spacer / 2)) {
   height: 0;
-  margin: ($spacer / 2) 0;
+  margin: $margin-y 0;
   overflow: hidden;
   border-top: 1px solid $color;
 }