Add no-unused-styles default import support

[aeirola]

This PR adds support for identifying unused styles when StyleSheet isn't imported separately, but instead used through the react-native default import, for example in

import RN from 'react-native';
const styles = RN.StyleSheet.create({});

Fixes #227
Replaces #218

[aeirola]

This can potentially be heavy operation. I wonder if it should be cached in some way. Also, maybe it should be moved as the last check?

[aeirola]

Did bump into some performance issues with unnecessary import checks. Decided to move it as the last check to improve performance.