Add stylelint for CSS linting (#122)

* Add stylelint

* Add lint-css package.json script

* Ignore all CSS files in public/build folder

.stylelintrc.json

@@ -0,0 +1,21 @@
+{
+  "extends": "stylelint-config-standard",
+  "ignoreFiles": ["public/build/*.css"],
+  "rules": {
+    "color-hex-case": null,
+    "color-hex-length": null,
+    "comment-empty-line-before": null,
+    "custom-property-empty-line-before": null,
+    "declaration-colon-newline-after": null,
+    "function-name-case": null,
+    "indentation": null,
+    "no-descending-specificity": null,
+    "number-leading-zero": null,
+    "rule-empty-line-before": null,
+    "selector-combinator-space-before": null,
+    "selector-list-comma-newline-after": null,
+    "selector-type-no-unknown": null,
+    "value-keyword-case": null,
+    "value-list-comma-newline-after": null
+  }
+}