Add tool 'box-shadow'

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

_scss/utilities/_boxshadow.scss

@@ -0,0 +1,13 @@
+// Box Shadow
+//
+// The box-shadow CSS property is used to add shadow effects around an element's frame. You can specify multiple effects separated by commas if you wish to do so. A box shadow is described by X and Y offsets relative to the element, blur and spread radii, and color.
+//
+// Values: offset-x | offset-y | blur-radius | spread-radius | color
+// Default: none
+//
+// https://www.w3.org/TR/css-backgrounds-3/#propdef-box-shadow
+@mixin boxshadow($value: none) {
+  -webkit-box-shadow: $value;
+     -moz-box-shadow: $value;
+          box-shadow: $value;
+}