Add support for specifying a KMS key for S3 storage.

[jgreat]

Right now S3 bucket encryption with KMS is limited to using the default key. Can the option for specifying a KMS key be exposed?

I believe this would require adding an option to specify s3manager.UploadInput.SSEKMSKeyId and making it available for configuration.

storage/amazon.go

Line 144 in ddb22a8

if b.SSE != "" {