Allow passing in custom controls via the message props?

[phillbaker]

Thanks for your work on maintaining this project!

What do you think about allowing custom controls in the message alert prop? So for example, to pass in buttons similar to the native push notification:

Example usage:

MessageBarManager.showAlert({
  title: 'Your alert title goes here',
  message: 'Your alert message goes here',
  alertType: 'success',
  children: <View style={{...}}><Button onPress={this.foo}>Not now</Button><Button onPress={this.foo}>Done</Button></View>,
});

I think this (very old) pr KBLNY#20 would allow this? It would probably need documentation updates and a rebase.

[phillbaker]

Bump @Talor-A would you accept a PR to do the above?