SampleApp: support cross-platform cp.

Author: peacechen

SampleApp/package.json

@@ -2,7 +2,8 @@
   "main": "node_modules/expo/AppEntry.js",
   "scripts": {
     "start": "expo start",
-    "postinstall": "cp -f ../*.js node_modules/react-native-modal-selector/",
+    "postinstall": "npm run cp",
+    "cp": "cpx \"../*.js\" node_modules/react-native-modal-selector/ -u",
     "nodemon": "nodemon nodemon.json",
     "android": "expo start --android",
     "ios": "expo start --ios",
@@ -20,6 +21,7 @@
   "devDependencies": {
     "babel-preset-expo": "~8.0.0",
     "@babel/core": "^7.0.0",
+    "@deboxsoft/cpx": "^1.5.0",
     "nodemon": "^1.18.10"
   },
   "private": true