Improve renaming implementation

[jmatsushita]

Hi again,

Right now running init allows to rename the app through a broad recursive string replacement in the ios, android folders and config files. I understand the thinking behind "exop-blank" but it's a bit scary :)

It would be less error prone to use something like https://www.npmjs.com/package/react-native-rename (and targeted key replacement in the config files). Or maybe target specific steps from this manual process or this one?