Adds no-unknown-custom-properties (#142)

Author: nikolai-shabalin

.stylelintrc

@@ -86,6 +86,7 @@
     "function-no-unknown": true,
     "media-feature-name-no-unknown": true,
     "media-feature-name-value-no-unknown": true,
+    "no-unknown-custom-properties": true,
     "property-no-unknown": true,
     "selector-pseudo-class-no-unknown": true,
     "selector-pseudo-element-no-unknown": true,