Make the library compatible with iOS and Android

[adrianicv]

In order to make the library compatible with iOS and Android I've included an index.native.js to export the FlatList and SectionList classes from react-native in case we are not running in a web environment (this is detected in package.json). The drawback is that we no longer can import from aliases, instead, we import directly from react-native-web-lists. If this isn't a problem I offer a PR to merge with master.

Thanks in advance.