add postinstall script

added a postinstall script which will build the styles.ts file when you
first clone the project. This step had to be done manually before and
lead to TS errors if you forgot it.

package.json

@@ -8,6 +8,7 @@
     "test": "tsc test/demo.ts --outDir docs --removeComments",
     "prepublishOnly": "npm run version",
     "version": "node util/copyrightHeader.js && npm run build && npm run test && git add *",
+    "postinstall": "node util/convertStyles.js",
     "build": "node util/convertStyles.js && rollup -c"
   },
   "repository": {