SCSS support tools

Signed-off-by: Valentin Popov <info@valentineus.link>

_scss/utilities/_boxsizing.scss

@@ -0,0 +1,15 @@
+// Box Sizing
+//
+// The box-sizing property in CSS controls how the box model
+// is handled for the element it applies to.
+//
+// Values: content-box | border-box
+// Default: content-box
+//
+// https://www.w3.org/TR/css-ui-3/#box-sizing
+@mixin box-sizing($value: content-box) {
+  -webkit-box-sizing: $value;
+     -moz-box-sizing: $value;
+      -ms-box-sizing: $value;
+          box-sizing: $value;
+}