Provide ability to create WARCs that are tar.gz

[machawk1]

As suggested by Noah Levitt @ internet archive.

[machawk1]

https://github.com/beatgammit/gzip-js might be relevant.

[kblumenthal]

I would like to create warc.gz files with WARCreate so that I may then ingest them into a storage repository that validates files by, among other things, the warc.gz or arc.gz file extension.