Allow more general write options for S3

[momolog]

This change allows for options like cache control headers to be set while writing to S3, e.g.

driver = Vos::Drivers::S3.new \
  access_key_id: aws_key, secret_access_key: aws_secret, bucket: bucket_name,
  write_options: {:cache_control => 'public,max-age=31536000'}

Please consider pulling this.