Update display.js

Add utility class for `display: contents`. This is useful for flex and grid. You can read more here https://css-tricks.com/get-ready-for-display-contents/

src/plugins/display.js

@@ -5,6 +5,9 @@ export default function() {
         '.block': {
           display: 'block',
         },
+        '.contents': {
+          display: 'contents',
+        },
         '.inline-block': {
           display: 'inline-block',
         },