Create npm prepublish script that builds the bundles before publish

Build the unminified and minified client-side bundles before
publish.

package.json

@@ -7,6 +7,7 @@
   "scripts": {
     "build": "NODE_ENV=development webpack index.js dist/html-to-react.js",
     "build-min": "NODE_ENV=production webpack -p index.js dist/html-to-react.min.js",
+    "prepublish": "npm run build && npm run build-min",
     "test": "mocha",
     "lint": "eslint index.js \"lib/**\" \"test/**\"",
     "cover": "istanbul cover _mocha -- -R spec \"test/**/*\"",