Allow more general write options for S3

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.