Allow prop-types rule to skip non-exported SFCs

From the docs:

For now we should detect components created with:

• React.createClass()
• an ES6 class that inherit from React.Component or Component
• a stateless function that return JSX or the result of a React.createElement call.

It would be great if we could enforce propTypes being defined on React.createClass() and React.Component-extended components, but not stateless functions which return JSX.

In fact, defining .propTypes in function use such as { arr.map((name, i) => <Row key={ i }>{ name }</Row>) } would increase verbosity considerably