App: Support setting defaultRemovalPolicy

[thdxr]

Closes #634

[fwang]

I think S3 buckets might be trouble b/c they need to be emptied before removed. You'd have to set autoDeleteObjects:

new s3.Bucket(this, 'MyBucket', {
  removalPolicy: RemovalPolicy.DESTROY,
  autoDeleteObjects: true,
});

And when autoDeleteObjects is true, CDK creates a custom resource to empty the bucket on stack remove.

I can't think of a good solution off the bat. We might not be able to set this flag right before synth. Not sure, maybe give it a try.