Add build command to package.json

Author: alizain

package.json

@@ -29,6 +29,7 @@
     "typescript": "^2.5.3"
   },
   "scripts": {
+    "build": "./node_modules/.bin/rollup -c",
     "test": "./node_modules/.bin/istanbul cover node_modules/mocha/bin/_mocha -- -R spec",
     "perf": "./node_modules/.bin/matcha perf.js"
   },