Add xs size prop to button

src/style/atoms/_button.scss

@@ -38,6 +38,18 @@
     /*
     ** Size variants
     */
+    // XSmall
+    &--xsmall {
+        height: 32px;
+        padding: 0 6px;
+
+        @include bp (sm) {
+            height: 32px;
+            padding: 0 16px;
+            font-size: rem(14px);
+        }
+    }
+
     // Small
     &--small {
         height: 40px;