Add additional section sizing

Author: rbrtsmith

demo/index.html

@@ -506,11 +506,21 @@ <h2>Section</h2>
         Sectioned content
       </div>
     </div>
-    <div class="c-demo-card">
+    <div class="c-demo-card u-push-bottom-md">
+      <div class="o-section-sm">
+        Sectioned content (small)
+      </div>
+    </div>
+    <div class="c-demo-card u-push-bottom-md">
       <div class="o-section-md@lg">
         Sectioned content at breakpoint
       </div>
     </div>
+    <div class="c-demo-card">
+      <div class="o-section-sm o-section-md@lg">
+        Sectioned content at breakpoint
+      </div>
+    </div>
   </section>
 
   <section class="o-section-md" id="flag">

nebula-css/_settings.scss

@@ -27,6 +27,7 @@ $nb-list-spacing: (
 ) !default;
 
 $nb-section-spacing: (
+  sm: $nb-spacing-unit,
   md: $nb-spacing-unit-double
 ) !default;