Border Box mixin

Signed-off-by: Valentin Popov <valentineus@gmail.com>

_scss/_tools.scss

@@ -0,0 +1,16 @@
+// 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 borderbox($value: content-box) {
+  -webkit-box-sizing: $value;
+     -moz-box-sizing: $value;
+      -ms-box-sizing: $value;
+          box-sizing: $value;
+}