Add custom properties version to build

Author: jxnblk

package.json

@@ -8,7 +8,9 @@
     "minify": "postcss -u cssnano css/basscss.css -o css/basscss.min.css",
     "important": "postcss -u postcss-importantly css/basscss.css -o css/basscss-important.css",
     "important:min": "postcss -u postcss-importantly -u cssnano css/basscss.css -o css/basscss-important.min.css",
-    "prepublish": "mkdir -p css && npm run build && npm run important && npm run minify && npm run important:min",
+    "custom-props": "postcss -u postcss-import -u postcss-custom-media src/basscss.css -o css/basscss-cp.css",
+    "custom-props:min": "postcss -u postcss-import -u postcss-custom-media -u cssnano src/basscss.css -o css/basscss-cp.min.css",
+    "prepublish": "mkdir -p css && npm run build && npm run important && npm run custom-props && npm run minify && npm run important:min && npm run custom-props:min",
     "start": "postcss -c postcss.config.js -w",
     "karma": "karma start",
     "karma:watch": "karma start --no-single-run",