add a generic media-query mixin

Author: monkeywithacupcake

_sass/_media-queries.scss

@@ -1,3 +1,9 @@
+@mixin media-query($device) {
+  @media screen and (max-width: $device) {
+    @content;
+  }
+}
+
 @media screen and (min-width: $viewport-small) {
   html {
     font-size: 16px;