feature request: add support for gzipped content

[vincentbernaud]

Hi,

I manage millions of small files that needs to be concatenated and your tool works great for that.
In the case of gzipped source files, I needed to decompress the file before doing the concatenation based on the ContentEncoding header in S3 response.

If you're interested, I can submit a PR with the changes to handle automatically gzipped content (It's only 6 lines of code).
Maybe an arg can be added in MultipartUploadJob if you wan't to enable it manually.

Thanks for this lib.