Support for ServerSide Encryption Flag

[dodgemich]

For security purposes, use of the server side encryption flag would be very handy as another option to pass in. Wasn't sure on preferred mode of injection, but seems like a simple change.

http://docs.aws.amazon.com/cli/latest/reference/s3/sync.html

[--sse ]
[--sse-c ]
[--sse-c-key ]
[--sse-kms-key-id ]
[--sse-c-copy-source ]
[--sse-c-copy-source-key ]

[fabiob]

@dodgemich, I would like to see a PR for that.

[lstellway]

I believe the S3_SYNC_FLAGS environment variable would be able to accomplish this (inline reference).