Add stylelint-scss plugin

This adds extra SCSS-specific rules. In particular, there's a modified at rule
processing rule which understands `mixin` and friends.

.stylelintrc.js

@@ -1,5 +1,8 @@
 module.exports = {
     "extends": "stylelint-config-standard",
+    "plugins": [
+        "stylelint-scss",
+    ],
     "rules": {
         "indentation": 4,
         "comment-empty-line-before": null,
@@ -11,5 +14,7 @@ module.exports = {
         "number-no-trailing-zeros": null,
         "number-leading-zero": null,
         "selector-list-comma-newline-after": null,
+        "at-rule-no-unknown": null,
+        "scss/at-rule-no-unknown": true,
     }
 }