Add bg-auto to the backgroundSize generator

Author: hacknug

src/generators/backgroundSize.js

@@ -2,6 +2,9 @@ import defineClasses from '../util/defineClasses'
 
 export default function() {
   return defineClasses({
+    'bg-auto': {
+      'background-size': 'auto',
+    },
     'bg-cover': {
       'background-size': 'cover',
     },