add prepare script

Author: drake.aren

package.json

@@ -8,7 +8,8 @@
     "watch-build": "npm run build -- -w",
     "test": "mocha --compilers js:@babel/register",
     "watch-test": "npm test -- -w",
-    "lint": "eslint **/*.js --quiet"
+    "lint": "eslint **/*.js --quiet",
+    "prepare": "npm run build"
   },
   "repository": {
     "type": "git",