Detect uninitialized states and statefulness

First things first, thank you for making this great plugin. 🙌🏻I'm hopeful that I will be able to contribute to this project. Maybe you can tell me if my two requests below make sense. Or maybe there are some rules which I do the job that I haven't discovered yet.

• PureComponent detection works well but not the other way around: there are no errors thrown for using setState in a component declared as extends React.PureComponent.
• In React 16.3, it eslint should warn if we have uninitialized state in a stateful component.