example postinstall improvements

[brody4hire]

• 1: use an external npm utility instead of examples_postinstall.js from the template to fix the npm package problem
• 2: [convenience] add a script such as prepare-ios that does cd ios && pod install
• 3: example postinstall script could automatically do cd ios && pod install (idea from fix generated iOS example on React Native 0.60(+) #99)
• 4: [convenience] add run-android & run-ios scripts that do react-native run-android & react-native run-ios

some additional points:

• Improvement 1 is basically a workaround for a limitation of npm and Yarn tooling. I think it would be really nice if the tooling would be fixed to detect and avoid this kind of problem.
• Improvement 3 could really benefit from the following proposal: https://npm.community/t/feature-request-prepare-script-that-does-not-run-before-publish/2498
• Using Yarn workspaces may be a better way to solve the problem addressed by improvement 1.

/cc @dsyne