Added script to check CSS files using stylelint.

.stylelintrc

@@ -0,0 +1,12 @@
+{
+  "rules": {
+    "media-feature-name-no-unknown": true,
+    "property-no-unknown": true,
+    "selector-pseudo-class-no-unknown": true,
+    "selector-pseudo-element-no-unknown": true,
+    "selector-type-no-unknown": true,
+    "string-no-newline": true,
+    "unit-case": "lower",
+    "unit-no-unknown": true
+  }
+}

run-stylelint.sh

@@ -0,0 +1,11 @@
+#!/bin/sh
+set -e
+
+STYLELINT=$(which stylelint || true)
+if [ -z "$STYLELINT" ]; then
+    echo "Can't find command \"stylelint\" in $PATH"
+    exit 1
+fi
+
+echo Checking stylesheets with $STYLELINT ...
+find -name "*.css" -print0 | xargs -0 $STYLELINT