Added render props

[dyst5422]

Allowing for render props. This should simplify a lot of the issues people have using refs.

Example usage:

<NotificationSystem>
  {({ addNotification }) => (
    <button onClick={() => addNotification({ message: 'New notification' })}>Add Notification</button>
  )}
</NotificationSystem>

[coveralls]

Coverage decreased (-79.04%) to 16.544% when pulling ef74578 on dyst5422:master into e2c4a14 on igorprado:master.

[dyst5422]

Very confused as to why the coverage went down. I just added a single test for the render props and changed nothing else

[dyst5422]

@igorprado Can I get some feedback on making this one happen?

[syndbg]

@dyst5422 Will get back to you.

[dyst5422]

Any update on this one?