copy flow definitions to dist folder

Author: koddsson

package.json

@@ -14,7 +14,7 @@
     "prebuild": "npm run clean && npm run lint && mkdir dist",
     "build-umd": "BABEL_ENV=umd babel src/index.js -o dist/index.umd.js",
     "build-esm": "BABEL_ENV=esm babel src/index.js -o dist/index.esm.js",
-    "build": "npm run build-umd && npm run build-esm",
+    "build": "npm run build-umd && npm run build-esm && cp src/index.js.flow dist/index.umd.js.flow && cp src/index.js.flow dist/index.esm.js.flow",
     "pretest": "npm run build",
     "test": "karma start test/karma.config.js",
     "prepublishOnly": "npm run build"