[Feature] Platform specific variables

[ste7en]

A feature that can be useful is to support platform-specific secrets by prepending a platform prefix as ANDROID_ or IOS_, and later stripping them at build time in order to reference both variations with the same name.
E.g.

ANDROID_API_KEY = my_android_key
IOS_API_KEY = my_ios_key

import Config from 'react-native-config'

const { API_KEY: apiKey } = Config;

PRs #753, #754, #755, #756 add this feature