Add prefer-flow-prop-types rule

I would love a rule for preferring Flow types instead of propTypes. Our team is migrating and it would be great to use the linter for this purpose.

For example, it would produce an error on the following code:

export default class Statements extends React.Component {
  static propTypes = {
    items: PropTypes.array.isRequired,
    fetchStatements: PropTypes.func,
    isLoading: PropTypes.bool
  };
  // ...
}

But this code would remain valid:

export default class Statements extends React.Component {
  props : {
    items: string[],
    fetchStatements?: () => void,
    isLoading?: boolean
  };
  // ...
}