Allow fill: none rather than requiring a variable

Author: thibaudcolas

config.js

@@ -66,6 +66,7 @@ module.exports = {
                     'inherit',
                     'transparent',
                     'initial',
+                    'none',
                 ],
             },
         ],

src/__snapshots__/semver.test.js.snap

@@ -232,6 +232,7 @@ Object {
         "inherit",
         "transparent",
         "initial",
+        "none",
       ],
     },
   ],